As I said this is only my opinion, actually more my observation, but I have not experienced any problems with my Abit BE6-II. I've run PIII-450s which were Slot 1s as well as various Celeron and CeleronIIs up to and including a CeleronII-700 Engineering Sample all of which were used with an Abit Slotket III. The 700 was run at 1050@1.75V for close to a year with no complaints, with 98, 98SE, ME, 2000, XP Pro, and a couple versions of Mandrake.
I realize it doesn't fill your requirement of onboard sound, but for next to nothing you can get a PCI soundcard that is far better than the onboard sound you will find on any Slot 1 motherboard, since onboard sound is only now starting to come up to the level of moderately priced soundcards.
Hope it gives you something to think about.