SPX is an audio file encoded and compressed using Speex codec. Speex is a part of OGG family of codecs and was developed specifically for compressing human speech with sampling rates from 8 to 32 kHz. Speex codec is used for storing audio and VoIP data, and is optimized for high-quality speech compression at bitrates from 2 to 44 kbps.

SPX files also offer addtional functions such as stereo encoding, noise reduction, echo cancellation, etc.