FLV is a Flash Video container used on popular hosting sites that stores audio, video and other data. It often uses h.264 or h.263 codecs for video storage and MP3 for audio. There are a lot of programs for both Windows and Mac operating systems that are able to open .flv files.

Opus is a lossy audio coding format developed by the Xiph.Org Foundation and standardized by the Internet Engineering Task Force, designed to efficiently code speech and general audio in a single format, while remaining low-latency enough for real-time interactive communication and low-complexity enough for low-end ARM3 processors. Opus replaces both Vorbis and Speex for new applications, and several blind listening tests have ranked it higher-quality than any other standard audio format at any given bitrate until transparency is reached, including MP3, AAC, and HE-AAC.