I've been using Aimersoft Video Converter... does that and more for audio and video files. Just select the portion of the media that you want to cut out (or keep) and it does the rest. Also converts between media types and formats quickly and easily. I got it as part of one of the Mac software bundle offers several years ago, and it's not failed me yet.
Audacity's awesome for audio editing, but be aware that it cannot edit MP3 and other compressed audio without introducing additional loss. If you want lossless editing of MP3 / M4A / etc, check out Fission. Not free, but does the job well.