File types other them .mp3 might not work with the embedded player, but can still be played by opening them in a new tab, also, people might be interested in uploading/downloading file types such as .ogg regardless of a preview.

7mb is all boards, and not by choice but because that is the highest file size our host allows for single uploads.

I had a problem once that I tried to upload a high-res image that was less than 7MB, and the site said it was too big. I reduced the file size a bit, but it was still too big, and I kept repeating the process until the site finally accepted a ~4MB file. This has happened more than once, but that's the only specific instance I remember.