If you mean specifically as the target of a post, you can post a link to whatever you want (well, at least an http/https link...I don't think that all protocols are supported, though I haven't checked).

However, I don't believe that the img-rs instance that Lemmy instances normally run to allow hosting images will accept mp3s. So you'd have to upload it elsewhere (catbox.moe or something like that) that can accept mp3s and then link to it.

checks

At least on lemmy.today, yeah, it doesn't accept it.

The only format that I know that supports audio that img-rs will accept is WebM, and only if it has a video component.

Combining an image and mp3 audio file into a WebM:

$ ffmpeg -i denver-international-airport-blue-mustang-statue-blucifer-1600x1067.jpg -i 11325622-tr707-crash-cymbal-241376.mp3 11325622-tr707-crash-cymbal-241376.webm

Yields (hosted on lemmy.today's img-rs):

The situation may be different for PieFed and for Mbin, and note that not all clients may be able to handle WebM, though the Lemmy Web UI is fine with it.