An audio editor might seem like one of those pieces of software that would be nice to have, but the costs would likely be too high to justify. After all, you might only want to edit an audio file once in a while. In my professional life, I only use it occasionally to clean up parts of voice messages from clients and to organize the audio recordings I have made at state hearings.
Thus, paying for audio recording software, in my professional life at least, would be a waste of money. That is why Audacity is such a great program. Audacity is an open-source program available for Windows, Linux, and Mac OS.
It allows for layering of various sound files. You can import from and export to a few different formats, including .mp3. Additionally, it provides a number of different preprogrammed effects. The one I found the most useful is the ability to eliminate background noise. I have used it a number of times in connection with the Android App I'm currently working on and found it did a good job. It would be incredibly useful for eliminating a hum or other audio artifact in a voice mail.
The user interface seems a little cluttered to me, though this is by no means a deal breaker. Additionally, in some of the effects, the options that you can set can be a little overwhelming.
Overall, if you need an audio editor, Audacity is where I would start.