I want to ask a question on the music stack exchange about my voice and I'd like to upload and MP3? Is this possible?
No. There is no support to upload files other than images (through Stack Exchange's own Imgur account). You can link the file though if you store it elsewhere. Maybe use an own online file storage service.
Make sure though not to breach copyright by making the file available for download. In some counties it might not be allowed to distribute it this way.