Audio format developed by the Nippon Telegraph and Telephone Corporation (NNT Labs); uses transform-domain weighted interleaved vector quantization to compress audio data; can be encoded in 80, 96, 112, 128, 160, and 192 kbps bitrates.

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.