Leopard would need Yikes and Gossamer Kexts copied in even to have a chance of booting. I have said Kexts, but they're extremely rare so I doubt he would be running 10.5 on it, and that doesn't even take into account the machine would need a G4 processor upgrade in it.

I have had the audio "buzzing" issue under just about every version of Mac OS X. It still happens (albeit very infrequently) on my Intel iMac. Irritating, and certainly a bug in the audio drivers, but not one that you should have to worry about. It doesn't signal impending hardware failure or anything of the sort.