photoshop is a bitmap (ie, image) editor, not a video editor. in addition, mp3's are music, not video files.
if you want to edit an mp3, use adobe audition. if you want to edit a video like an mp4, use adobe premiere.
but if you want to edit in photoshop use 'motion' and any of the following formats:
MPEG‑2 if a decoder is installed (for example, with an Adobe video suite)