Possible issues with IRQ, maybe?

I posted here awhile back, I was rebuilding my system into the
following:

Soyo Dragon Lite mobo
P4 2.8GHz CPU
512MB Kingston RAM
40G Western Digital HDD
Radeon 9700 Pro video card

Unfortunately, I had to run Win98FE for awhile, and still am running
it. I may upgrade to Win2003, but I'm still undecided.

A few weeks after the upgrade, I've had some problems. For what
appeared to be no reason, my US Robotics 56k modem started connecting
VERY slowly every night after 11pm (my laptop connects fine, so it's
not the phone line or the ISP). Then a few days ago, I noticed that
some of my sound files aren't working anymore; for instance, one .wav
file will work fine, but another just doesn't make any sound (no
error, just no sound; same goes for .mpg and .avi files).

The hard drive and sound card is the same as the old system, and I
didn't have any problems with either before the upgrade. But I noticed
that both modem and sound card are currently on IRQ 11 (as well as the
Radeon 9700). I had changed the modem to a different IRQ with no
effect, but I can't help but notice that the two new problems are on
the same IRQ.

I reinstalled the modem with no effect, but haven't done anything with
the sound card; I didn't want to risk losing the few sounds that I
still have.

Is it just a coincidence that these two things are on the same IRQ? To
be honest, I really have no idea what the IRQ is. Is this something
that I can reinstall?

Any other thoughts?

TIA,

Mike