I found that the latest version of Audacity will open and edit the .mus music files used by Need for Speed III Hot Pursuit, however there does not seem to be an option to then export the edited .mus file.
The Mus format appears to also have been used by some other old games like DOOM.
Is there any way to or plug in that will allow Audacity to export this format, or a ogg or mp3 to mus conversion utility?
According to this fandom website, the MUS format is a slightly altered MIDI format used in the Doom engine soundtracks.
Audacity does not support the MUS format, but it can export MID (MIDI) format from Note Tracks.
If the MUS file imports as a Note track, then you can export it as a MID file.
If the MUS file imports as an audio track, then either that website is wrong, or FFmpeg is reading it as a MOD (tracker) file.
Tracker files are similar to MIDI files but usually they contain audio samples that are then used to render the notes as audio. MIDI files do not contain audio, just note data, so they generally sound pretty bad when converted by FFmpeg.
Audacity cannot convert audio tracks to MIDI, but can export audio in a wide range of audio formats, including WAV, OGG, FLAC, AIFF, MP3 (requires that LAME is installed).