Audio unstable

Having audio problems while playing avi's in windows media player 9 (had v11 and rolled back to v9 thinking the problem would be solved). The sound fluctuates from very, very loud to almost unhearable .. and during all this time I can hear all the background noise so clear. It's like the sound problem is only with the voices. Don't laugh, I think it's crazy too.

Thinking it might be a codec problem, I used a program to see if I had all the codecs I needed. It found one problem:
WMAPro over A/PDIF DMO ..... says it is "broken" and there is no driver installed for this codec.

I have searched and can't find any answer to my problem and I really could use some help. I keep my hand on the volume control in hopes that someday this problem will be solved.

Info on my computer ... if you need more, please ask ... Thank you
XP Pro SP3
Intel Core2
2.4GHz
3.25G Ram
Sigma Tel Audio (onboard sound)

Thank you

 

I could only find WMAPro over S/PDIF DMO, can you check the error again just to make sure.

Also do you have any other media players installed, I know there is a strong possibility this is just a Media Player error but, but I just wanted to rule out any other issues.

 

Sorry about that ... it was a typo .... WMAPro over S/PDIF DMO

I have no other media players installed.

Thank you for answering me.