PsychoPy supports various sound engines. The main ones are using a library called libsndfile (http://www.mega-nerd.com/libsndfile/) to load files from disk and this doesn’t support mp3. It does support a variety of other options (ogg, flac, aiff) but mp3 is a proprietary format that would require fees to be paid. As this (PsychoPy, pyo, libsndfile) is all free software we have no way to pay those fees.