One big advantage of the mac mini is that the power supply is separate from the computer, this makes for a very quite (electrically speaking) set up.
Secondly for the most part the Mac mini runs silently in the audio sense, it is rare indeed that you would hear the fan at all in normal operation.
About the only downside is limited HD space, but ideally you should be using an external drive dedicated to audio, and you can even get a matching drive!
Join the cult of the mac mini, the other solutions are really pretty clunky by comparison.