Did you make core code edits for the MP3 player? If so that's probably what the issue is, you've replaced files that had edits in them during the update, try re-doing the edits on your new files. Incidentally, this is exactly the reason core code editing isn't recommended with 3.1, that's what extensions are for. If you do edit your files again you will have the same issue next time you update.
You say "upgrade" do you mean "update" there is a difference? An "upgrade" would signify your previous version was 3.0.
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