You can use your own ringtones so you can match your jingle to your mood, or match just the right tune to your contacts. Just make sure the audio file is:
To make any file that fits these criteria into a ringtone, all you have to do is assign it the genre of ringtone. If you have a PC, you'll use the Zune software. If you don't have the Zune software on your computer, see Install the Zune software. On a Mac, use iTunes to assign the genre, and then sync the file using the Windows Phone Connector for Mac. To learn more, see Sync and get updates with my Mac.
To see the length, size, and genre of a file in the Zune software, go to Collection > Music > Songs, then click CHOOSE COLUMNS. Select the Genre, Length, and Size check boxes, then click OK.
Learn More: http://www.microsoft.com/windowsphone/en-us/howto/wp7/start/create-ringtones.aspx