Convert any video to MP3
Drop an MP4, MOV, MKV, WebM, or MPEG-TS file and download an MP3 of its audio at 192 kbps. The video is discarded, and the sample rate and channel count of the source carry through.
What the file size tells you
A 3 GB video does not make a 3 GB MP3. Video is nearly all of the bytes in a media file; the audio is usually 5 to 10 percent of it. Ninety minutes at 192 kbps lands near 130 MB whatever the picture was.
The read still has to walk the container to find the audio packets, so a large file takes longer to get through even though almost none of it is decoded.
Formats that will not open
AVI, WMV, and FLV have no demuxer here and are refused at the file picker. They are old container formats, and the codecs inside them, especially the audio, often have no browser decoder either.
MKV is the other place this stops: AC-3, E-AC-3, and DTS audio cannot be decoded, which rules out most film rips. The codec is reported as soon as a file loads.
Skip the re-encode where you can
An MP4’s AAC track can be copied straight into an M4A with no quality cost at all. MP3 is worth its second lossy pass only when the destination insists on it.