In reply to:

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Anyway, I've never done it, but it seems to me playing an MP3 backwards would just be an exercise in starting at the end, decoding the last frame, reversing it, and lining it up with the "next" (actually previous) frame in the output stream.

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It's a bit harder with VBR files than forwards decoding, as you need to search for the beginning of each frame. In effect, you're always unsynchronised with the stream.

I think you could probably reverse an MP3 file losslessly without fully decoding it, now I think about it (and you can parse it forwards if you're willing to write the output backwards).