The WinMM multimedia APIs include an API for enumerating and controlling all the paths through the audio device; things like bass boost, treble control, pass-through audio from your CD player to your headphones, etc.  This is called the "mixer" API and is the forerunner of the IDeviceTopology API.

I wrote a quick app to enumerate all the mixer devices on the system; for each mixer device, enumerate each mixer line (that is, each source and destination); for each mixer line, enumerate all the controls (volume, mute, equalization, etc.); and for each control, query the associated text (if any) and the current value.

Source and binaries attached.

Pseudocode:

mixerGetNumDevs()
for each mixer device
    mixerGetDevCaps(dev)
    for each destination (line) on the device
        mixerGetLineInfo(dest)
        mixerGetLineControls(dest)
        for each control on the line
            if the control supports per-item description
                mixerGetControlDetails(control, MIXER_GETCONTROLDETAILSF_LISTTEXT)
                log the per-item description
            mixerGetControlDetails(control, MIXER_GETCONTROLDETAILSF_VALUE)
            log the value(s)

Usage:

>mixerenum.exe
Mixer devices: 5
-- 0: Contoso headset --
    Device ID: 0
    Manufacturer identifier: 1
    Product identifier: 104
    Driver version: 6.2
    Product name: Contoso headset
    Support: 0x0
    Destinations: 1
        -- Destination 0: Master Volume --
            Destination: 0
            Source: -1
            Line ID: 0xffff0000
            Status: MIXERLINE_LINEF_ACTIVE (1)
            User: 0x00000000
            Component Type: MIXERLINE_COMPONENTTYPE_DST_HEADPHONES (5)
            Channels: 1
            Connections: 2
            Controls: 2
            Short name: Volume
            Long name: Master Volume
            -- Target:  --
                Type: MIXERLINE_TARGETTYPE_UNDEFINED (0)
                Device ID: 0
                Manufacturer identifier: 65535
                Product identifier: 65535
                Driver version: 0.0
                Product name:
            -- Control 1: Mute --
                Type: MIXERCONTROL_CONTROLTYPE_MUTE (0x20010002)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Mute
                Long name: Mute
                -- Values --
                    FALSE
            -- Control 2: Volume --
                Type: MIXERCONTROL_CONTROLTYPE_VOLUME (0x50030001)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Volume
                Long name: Volume
                -- Values --
                    0xffff on a scale of 0x0 to 0xffff
-- 1: HDMI Audio (Contoso --
    Device ID: 1
    Manufacturer identifier: 1
    Product identifier: 104
    Driver version: 6.2
    Product name: HDMI Audio (Contoso
    Support: 0x0
    Destinations: 1
        -- Destination 0: Master Volume --
            Destination: 0
            Source: -1
            Line ID: 0xffff0000
            Status: MIXERLINE_LINEF_ACTIVE (1)
            User: 0x00000000
            Component Type: MIXERLINE_COMPONENTTYPE_DST_DIGITAL (1)
            Channels: 1
            Connections: 2
            Controls: 2
            Short name: Volume
            Long name: Master Volume
            -- Target:  --
                Type: MIXERLINE_TARGETTYPE_UNDEFINED (0)
                Device ID: 0
                Manufacturer identifier: 65535
                Product identifier: 65535
                Driver version: 0.0
                Product name:
            -- Control 1: Mute --
                Type: MIXERCONTROL_CONTROLTYPE_MUTE (0x20010002)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Mute
                Long name: Mute
                -- Values --
                    FALSE
            -- Control 2: Volume --
                Type: MIXERCONTROL_CONTROLTYPE_VOLUME (0x50030001)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Volume
                Long name: Volume
                -- Values --
                    0xffff on a scale of 0x0 to 0xffff
-- 2: Speakers (Contoso --
    Device ID: 2
    Manufacturer identifier: 1
    Product identifier: 104
    Driver version: 6.2
    Product name: Speakers (Contoso
    Support: 0x0
    Destinations: 1
        -- Destination 0: Master Volume --
            Destination: 0
            Source: -1
            Line ID: 0xffff0000
            Status: MIXERLINE_LINEF_ACTIVE (1)
            User: 0x00000000
            Component Type: MIXERLINE_COMPONENTTYPE_DST_SPEAKERS (4)
            Channels: 1
            Connections: 2
            Controls: 2
            Short name: Volume
            Long name: Master Volume
            -- Target:  --
                Type: MIXERLINE_TARGETTYPE_UNDEFINED (0)
                Device ID: 0
                Manufacturer identifier: 65535
                Product identifier: 65535
                Driver version: 0.0
                Product name:
            -- Control 1: Mute --
                Type: MIXERCONTROL_CONTROLTYPE_MUTE (0x20010002)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Mute
                Long name: Mute
                -- Values --
                    FALSE
            -- Control 2: Volume --
                Type: MIXERCONTROL_CONTROLTYPE_VOLUME (0x50030001)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Volume
                Long name: Volume
                -- Values --
                    0xffff on a scale of 0x0 to 0xffff
-- 3: Contoso headset --
    Device ID: 3
    Manufacturer identifier: 1
    Product identifier: 104
    Driver version: 6.2
    Product name: Contoso headset
    Support: 0x0
    Destinations: 1
        -- Destination 0: Master Volume --
            Destination: 0
            Source: -1
            Line ID: 0xffff0000
            Status: MIXERLINE_LINEF_ACTIVE (1)
            User: 0x00000000
            Component Type: MIXERLINE_COMPONENTTYPE_DST_WAVEIN (7)
            Channels: 1
            Connections: 1
            Controls: 2
            Short name: Volume
            Long name: Master Volume
            -- Target: Contoso headset --
                Type: MIXERLINE_TARGETTYPE_WAVEIN (2)
                Device ID: 0
                Manufacturer identifier: 1
                Product identifier: 101
                Driver version: 6.2
                Product name: Contoso headset
            -- Control 1: Mute --
                Type: MIXERCONTROL_CONTROLTYPE_MUTE (0x20010002)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Mute
                Long name: Mute
                -- Values --
                    FALSE
            -- Control 2: Volume --
                Type: MIXERCONTROL_CONTROLTYPE_VOLUME (0x50030001)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Volume
                Long name: Volume
                -- Values --
                    0xf332 on a scale of 0x0 to 0xffff
-- 4: Microphone (Contoso --
    Device ID: 4
    Manufacturer identifier: 1
    Product identifier: 104
    Driver version: 6.2
    Product name: Microphone (Contoso
    Support: 0x0
    Destinations: 1
        -- Destination 0: Master Volume --
            Destination: 0
            Source: -1
            Line ID: 0xffff0000
            Status: MIXERLINE_LINEF_ACTIVE (1)
            User: 0x00000000
            Component Type: MIXERLINE_COMPONENTTYPE_DST_WAVEIN (7)
            Channels: 1
            Connections: 1
            Controls: 2
            Short name: Volume
            Long name: Master Volume
            -- Target: Microphone (Contoso --
                Type: MIXERLINE_TARGETTYPE_WAVEIN (2)
                Device ID: 1
                Manufacturer identifier: 1
                Product identifier: 101
                Driver version: 6.2
                Product name: Microphone (Contoso
            -- Control 1: Mute --
                Type: MIXERCONTROL_CONTROLTYPE_MUTE (0x20010002)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Mute
                Long name: Mute
                -- Values --
                    FALSE
            -- Control 2: Volume --
                Type: MIXERCONTROL_CONTROLTYPE_VOLUME (0x50030001)
                Status: MIXERCONTROL_CONTROLF_UNIFORM (0x1)
                Item count: 0
                Short name: Volume
                Long name: Volume
                -- Values --
                    0xf332 on a scale of 0x0 to 0xffff