It handles separate channels, with separate volume for each. The problem you will run into is lack of drivers for all audio formats, but you could use some utility to temporarily convert them to wav or mp3.

Another big project going now, is gstreamer , but it is significantly more complex than SDL. There is a Perl module for gstreamer, but is poorly documented. So for something quick and easy, use SDL, but for a long term major project, use gstreamer. Gstreamer has a good maillist where you can find answers, and if you need some help putting sliding volume controls on my Tk app, I can help you with that. It ought to be pretty easy. Remember, both SDL and Tk are event-loop systems, but you can use Tk's event-loop to handle SDL events, so it's a good match.

P.S. SDL is probably easier cross-platform, since ActiveState has a module for it (for windows), whereas Gtk2 is still a bit tricky to make work on windows.

I've done some audio programming, but I skipped on the hard parts and just used the fmod library. This was on Windows, but likely you'd be able to write an XS wrapper to make fmod do your bidding. I just loaded the fmod.dll via Win32::API, which isn't that portable.

From the sox README:
SoX (also known as Sound eXchange) translates sound files between different file formats, and optionally applies various sound effects.
SoX is intended as the Swiss Army knife of sound processing tools. It doesn't do anything very well, but sooner or later it comes in very handy.

Sox is also recommended in the TODO section of Audio::MPEG.

If you need more effects than the batch processes that sox provides, you probably need a good UI - use Audacity.