The Sase libraries fron Nellysoft offer flexible compression options for both embedded and data packets based compression implementations It can operate in either a single bit rate or a multi-rate mode, offering compression rates (1.8/3.2/6.4 Kbps) to suit your application or the changing environment of packet based networks. Sase has the ability to switch bit rates on-the-fly to better handle changing network condition. At 3.2 Kbps Sase offers near toll quality, as you would expect when more bits are added the speech quality improves at 6.4 Kbps.

http://www.nellymoser.com/products/compression_fst.htm