Not sure about the Windows platform, but there ARE programs to directly edit mp3's WITHOUT conversion on the Mac. Probably, if you look hard enough, you can find a Windows equivalent.
And I HIGHLY recommend that you DO find one of these, because every time you convert to WAV and back for an edit, you lose a little bit more fidelity. They sound bad enough already, without you adding to the problem!
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An arranger is just a tool. What matters is what you build with it..!