Quicktime pro allows you to do this. You can set the begin and end markers and then select "save as" . It takes 2 steps in total.
1. Save shortened song as .mov (.mov is the only file type option offered at this stage)
2. Export this new mov file as an audio file
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I would recommend Audacity though for trimming songs. Its designed for these type of use cases. Here's the official site link:
http://audacity.sourceforge.net/download/mac