Unfortunately, I was running Jaguar (you know, CD magically appeared on my desk, etc.) and had a bunch of problems insmod'ing the new kernel drivers due to incompatibilities. So I thought I might as well go back to OS X 10.1.3.
Unfortunately, OS X won't downgrade itself on install. So it was going to be a hard install. No problem, I wanted to churn up the disk anyway, since I only had 100M free and could do with getting rid of the cruft. 10G of MP3s went to a spare partition on a cow-orker's machine. (He's off sick, and I'll get them off before he notices. Heh, heh...) 1G of home directory stuff went onto my desktop; around 3G of cruft got deleted. Now, by my reckoning, that adds up to 14G.
I do the reinstall, trashing the filesystem, copy back the 1G of ~simon, reinstall the applications that I use, and somehow there's 26G free. Something got away somewhere.
And for some reason, not only do I have more disk space, but the laptop's running much faster (maybe that was just Jaguar being slow, to be honest) and even more bizarrely, I have much better Airport range at home.
There's obviously something tremendously therapeutic about newfs'ing your hard disk and starting again.
Still haven't got the speakers working, though.