What is Icecast, the program? Icecast is a streaming server, which can stream audio (and video) to listeners/viewers. It supports Ogg (Vorbis, Theora), Opus, FLAC and WebM (VP8/VP9), nonfree codecs/formats like MP4 (H. 264, MPEG4), M4A, NSV, AAC and MP3 might work, but we do not officially support those.