ATTICUS X  This has become the IDEAL application for simple audio editing and converting sound files. In addition to its standard editing and recording functionality, Amadeus Pro lets you apply custom fades, add a variety of effects (including most VST and Audio Unit effects), insert text markers for your own reference, analyze and change various characteristics of your audio (bit rate, stereo/mono, etc), and a lot more. The batch processor saves time when you're performing the same operation(s) on many audio files. In fact, the whole interface is designed for efficiency: elegant, intuitive, streamlined... and very stable. For advanced multitrack editing you may prefer to use a more advanced multitrack editor (e.g., Logic, Ableton Live, Pro Tools), but for most everyday editing, I find Amadeus preferable. The developer is active and responsive to queries. The manual (accessible from the Help menu) is thorough and simple to understand. When troubleshooting becomes necessary, the user forum on the developer's website can be a great help. Amadeus is more than worth the low price and is altogether an essential piece of software for anyone that edits audio on a Mac.  
(Version 1.4)