What are SPX files? How to play SPX files?

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 revised BSD license.
The Technology

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)
• Fixed-point port
• Acoustic echo canceller
• Noise suppression

Note that Speex has a number of features that are not present in other codecs, such as intensity stereo encoding, integration of multiple sampling rates in the same bitstream (embedded coding), and a VBR mode.

One of the simplest things you can do to get involved in Speex is by using it in your application; Speex is well-suited to handle VoIP, internet audio streaming, data archival (like voice mail), and audio books. Currently, LinPhone, Ekiga, and Asterisk are some of the projects using Speex.