Yes, you can upload mp3s on the forum. It is certainly fine to upload an mp3 to get feedback or demonstrate something. With that said, I suspect the space on this forum costs Ben money without any direct income from it, so we often link to other sites where we are doing things that will involve a significant amount of files. For example, when working on a collaborative recording, we use Kompoz as a “home base” for all the mp3s. Some people use soundclick for storage and link to that. In short, I would say have at it and upload mp3, but if there are going to be a significant amount of files, maybe store them externally and link to them on the forums. I am looking forward to hearing them.