Mac: If you don't want to use iTunes, OS X doesn't really provide an easy way to convert audio formats. There are a few apps out there, but most don't work that well or cost money. Audio Converter Lite is free and can make quick work of conversions with just a drag and a click.
You take the files you want to convert and drag them onto the window. After that, you choose a preset and click convert. The files will end up in the specific folder and that's all there is to it.
The "lite" version will convert your audio to M4A, Mp4, CAF, ALAC and iPhone ringtones. The pro version ($7.49) will open up the app to custom settings, but the most common conversions are ready and available in the free app. Although not the world's most elegant software, it's very easy to use and handles conversions quickly. Not a bad deal for no money.
Audio Converter Lite (Free) [iTunes App Store]