The mp3 file might be an AAC file but renamed to mp3, or perhaps a corrupted mp3 file. Can you upload the mp3 file somewhere and put the link here so we can check?
You can send it by uploading it up to a file sharing service and then by sending the link here. Alternatively, you can upload the beatmap to the osu! servers/website by using the BSS and we can download it from there and see if it is truly mp3 or AAC.