From Hydrogenaudio Knowledgebase
Speex is an Open Source/Free Software patent-free audio compression format designed for speech. The Speex Project aims to lower the barrier of entry for voice applications by providing a free alternative to expensive proprietary speech codecs. Moreover, Speex is well-adapted to Internet applications and provides useful features that are not present in most other codecs. Finally, Speex is part of the GNU Project and is available under the Xiph.org variant of the BSD license.
 Technical Information
Speex is based on CELP and is designed to compress voice at bitrates ranging from 2 to 44 kbps. Some of Speex’s features include:
- Narrowband (8 kHz), wideband (16 kHz), and ultra-wideband (32 kHz) compression in the same bitstream
- Intensity stereo encoding
- Packet loss concealment
- Variable bitrate operation (VBR)
- Voice Activity Detection (VAD)
- Discontinuous Transmission (DTX)
- In-progress fixed-point port
- Echo Cancellation (experimental)
- Vorbis psychoacoustics model (experimental)
Note that Speex has a number of features that aren’t in other codecs such as Intensity stereo encoding, integration of multiple sampling rates in the same bitstream, and a VBR mode.
This information is copied from the Speex home page