You can generate MP3 audio from .mu3 music file in Harmony Assistant. This will generate a MIDI like music as playable .mp3 file. We do not know about any other program capable of mu3 to mp3 conversion, although some users mistype m3u to mp3 conversion, which is actually not doable.
Convert Harmony Assistant user instruments file to Compressed audio and music file, mp3 songs or ringtones.
We have found one software suitable in our database for this conversion.
Multiplatform software - convert mu3 to mp3 on any platform
A music composition and editing program
Convert from mu3 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.