This is a non-profit website that I work on in my free time. The code is public so anyone can view it. Here's the link to the GitHub repository. Contributors are welcome.
Convert an audio file to another format - MP3, AAC, WAV, Opus, Vorbis (.ogg), FLAC, ALAC, DTS, AC3 (Dolby Digital) or CAF.
Convert a video to an audio-only file (to any of the above formats).
Convert a video to the MP4 or MKV format.
Change the audio codec of a video to AAC, AC3, DTS, WAV, FLAC or ALAC.
Trim a video or audio file (will not work if using the Safari browser).
This website uses the latest version of the LAME encoder (v3.100) to convert a video/audio file to MP3. The LAME encoder is considered to be the highest quality encoder for MP3.
AAC is the codec of iTunes music and is the most common audio codec in MP4 videos. The extension of a raw AAC file is ".aac", and an AAC file wrapped in the MPEG-4 container has the extension ".m4a" (e.g. music downloaded from iTunes).
Vorbis is a lossy codec that performs well, and compares favourably to MP3 at low bitrates (source). This website saves files converted to Vorbis in the Matroska container [file extension: .mka].
Opus is the talk of the town in world of lossy audio codecs. It compares favourably to all of the major lossy codecs at low bitrates (source). This website uses the latest version of Opus; libopus 1.3.1