Mac OS X: You can convert audio files on your Mac with iTunes or a few other third-party applications, but there are often unnecessary steps involved and certain formats aren't supported. xACT is a very simple app without those problems and just gets the job done.xACT is just a graphic user interface (GUI) for a few command line tools, like LAME (MP3 encoder) and shntool (wave data processing tool), so it doesn't look or sound all that exciting. Its interface may also be a little confusing if you're not familiar with the command-line utilities it's built upon.
Nonetheless, once you get past any minor confusion you'll find that xACT is just really good at converting your audio files. It can handle a bunch of different formats, like SHN, FLAC, AIFF, OFR, LPAC, WV, ALAC, LA, TTA, TAK, MKW, MPEG-4 ALS and Bonk (to name a few — but not all).
Aside from working with a bunch of audio formats most GUI conversion apps won't touch, it's perhaps the easiest way to convert your audio files to AAC or MP3. Sure, you can do this in iTunes but then you have to add the original format to iTunes first. Other apps support this kind of conversion, but it's generally not as straightforward as using xACT. What's great about using xACT for these simple conversions is that you just add the files you want to convert, choose a format, and click the encode button. (Note: you'll find AAC and MP3 in the "lossy" tab rather than the "encoding" tab, since encoding seems to only refer to lossless formats.) You can adjust a few settings here and there, but the point is that you can create a batch conversion and start it in just a few seconds. It's really just the simplest way to convert your audio, whether it's to one of the popular lossy formats or one of the more rare lossless ones.
xACT is a free download. Currently you can grab a copy over at MacUpdate.