Slight volume wave with onboard sound

not a big deal with full albums, but single song mp3s, have an audible volume wave (slight, but just enough to hear).
im trying to use either asus onboard sound, or the sound 'evidently' coming from my xfx4850. ive tried to turn off either, both, and neither. lol. never had 2 sound sources in the hardware manager. but now i do. what it really sounds like is that whatever is providing sound is on a limited power supply, and struggling against its limitations. like its being kept back. and the volume dips just slightly as it struggles.
i wondered about driver conflicts, but if i turn either sound source off, i get no sound.
im about to see if my old soundblaster 5.1 will do anything. trying to run this rig without a soundcard tho.


build-
pc power and cooling 500w silencer
amd phenom 9600 quad
xfx hd4850 xxx
asus m3a76-cm mobo
4 gb ballistic
vista home premium

and just to note, sound comes through perfect for movies, streaming vid, whatever, and to my knowledge none of the song files were 'volume leveled', they all played loud and clear on my old xp system.
-any help or thoughts would be appreciated.
-fugfar

the solution for me was to disable one of the audio devices, and so far so good. i had to tweak a bit in control panel, but ive got it mostly solved.
what i mean by a volume wave, is imagine the strongest mp3 tune you have, but every 2-3 seconds the volume, and all frequencies fade like 5% just barely enough to make you think youre hearing things, then repeat, every 2-3 seconds. lol.
power fluctuations is what it sounds like, but its on a ups, andf i have no way of knowing what kind of power my mobo's audio is getting. problem is mostly solved. some older mp3s suffer, but everything older was transfered from old machine, all replaceable