An SPX file is a Speex audio file saved within an Ogg Vorbis container. Speex is an open source patent-free audio format designed for speech compression. It supports variable bitrate (VBR) encoding and several features not found in other audio codecs. Some examples include embedded encoding, intensity stereo encoding, and voice activity detection.

PVF is a special low-bandwidth ADPCM-based digital audio format for recording and storing human speech. A .pvf file contains digitally recorded speech in the PVF format. Such .pvf files were mainly used with legacy voice modems and software-based answering machines. Often, to be used with a specific modem, a .pvf file would have to be converted into the device-specific RMD format. Currently, PVF audio files can still be played with several major media players as well as converted into other formats (like WAV) with multi-format converter tools.