EasyAudio

The EasyAudio ActiveX Control adds speech handling capabilities to your Win32 program. Listed features include nice things such as support for popular codecs including G.729, G.711, and ADPCM, unicast and multicast support, AGC, jitter buffer management, and noise reduction. The web page gives a price of $1000 for the control, and $2000 for the source code. I hope that big price tag includes free distribution rights, but the web site is woefully short on license information.
Version 3.0 is shipping as of March, 2003.