My guess is that a network person with even the most basic skills could determine bandwidth usage, port, source and recipient, and probably file names.

Not that fie names tell the whole story. I think it's the case that even though a file might be called first_quarter_worksheet.xls it is possible that it is not a spreadsheet but in fact jennajamisonxxxporn.mpeg or some such.

My point being that in your case the files will more than likely have a .mp3 extension and be fairly identifiable as something which might not be work related and as such might warrant further scrutiny- especially if a lot of bandwidth is being utilized.