Is there a way to convert .apu files to .mp3 format?
We are not aware of any possible way to convert APU files directly to MP3 Audio Files (.apu to .mp3 conversion).
Perhaps this was just a typo, and you were, in fact, searching for ways to convert Audacity Project Files to MP3 Audio Files (.aup to .mp3 conversion). In such case, please refer to the AUP to MP3 conversion.
In a word, direct apu to mp3 conversion is not possible.
So don't expect any so-called apu to mp3 converter or apu to mp3 free online conversion tool available.
A free, open source software for recording and editing sounds