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If you want a free one try audacity, though it's not esspially good on the Mac compared to the windows version.

Also you can do it with gurageband I think, just drag the file directly from the folder into the app. This is probably a better option then audacity as it's got a much better interface however you can't apply filters to the sound like you can in audacity so depends what you need to do I suppose. Course both aren't really pro apps it all depends what you wana do!
 
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