Taking into account its W98 and age of laptop I wouldn't bother too much searching out the right drivers if it does what you want. Graphics you have sorted. Can't find any detailed specs for it but looks like its a similar machine (Advent, PIII, W98) to one that was kicking about at work so would be very surprised if it had wireless network or bluetooth. Possibly not even a wired network either.
As you've managed to install W98 can assume the CD (DVD?) drive is working.
Think that leaves sound. If you are hearing the windows startup music it will be using a generic driver and I'd leave it at that if its working.
There is no support for win98 now, think it stopped around 3 - 4 years ago. Service packs probably still available from MS website.
I wouldn't expect much from it, email, surfing, run an older copy of office. Laptops are now much more consumer items than when it was built as a business tool.
The correct drivers may give more features and work a little faster of course.
I wouldn't flash the bios. As Oddjob posted there is a risk involved and its unlikely you'll notice any difference anyway. Its an older laptop, so if its got to its present age without it why start fiddling with it now?
OR...if you fancy having a play give Linux a try. Ubuntu can run from a LiveCD so you can get a feel for it without having to install it. It's the only distribution I've used so others will be better placed to advise if its suitable for your use.
