-> Quicktime Pro. Best way out there, then it works in many other Apps (like SoundStudio), too... Buy it for like 30 bucks from Apple or PM me if you already bought it and lost your registration key...
actually, there is a version of musicmatch for osx if you check versiontracker.com, but while you're there, just search for "mp3" and you'll find a ton of utilities to convert audio. i think that audacity (http://audacity.sourceforge.net) will do this as well as allow you to edit/record audio, but there are several simpler applications to do this.