There are several good options for playing MP3s. For no-frill thrills, try MpegDec. It gets the job done without chewing up too much CPU power, letting you do other things with your 'book.

Another option is Audion, which used to be payware, but is now freeware, thanks to the generosity of its creators. It has more eye candy than MpegDec, and more features, but at the expense of chewing up more CPU cycles. Download both, try them out, and see what you like. Luckily, it's not an either-or decision; I have both on my 3400.

There are others (and I'm sure you'll hear about them shortly), but those two are my faves for older macs.