We periodically get asked where someone can download the SAPI 5.3 SDK.

For SAPI 5.0 & 5.1, the SDK was a standalone download.  It is now part of the platform SDK.  So, all you need to do is go to http://msdn.microsoft.com and search for what you are looking for.  For example, if you search for "SAPI 5.3", the first result returned is the overview page for SAPI 5.3.  There is a discussion on what is new in SAPI 5.3, and then of course all the API documentation and samples are there.