For years, SRS has been working miracles, creating Virtual Surround Sound from ordinary speakers. Now SRS technology is available as a Mac iTunes plug-in called iWow, which you can try for free. (There's a PC program available, too.)
Once I tried iWow on one of my Macs, I immediately bought it for all of them. It may be all in my head (literally), but it truly does make the sound richer and more satisfying. It was also the answer to a prayer: one of the recent OS X updates rendered my Macbook speakers practically inaudible. This simple plug-in solved the problem... at least for iTunes.