no sound msn messenger 6.1.0207

hi
i over the xmas holiday my msn messenger all of a sudden
stopped making a noise when ever my contacts try to talk
to me. it doesnt make a noise when they sign in or send
me an email either.
i cant rememeber when this started so i dont know if its
because i updated my msn messenger 6 to 6.1.
does anyone else have the same problem? can anyone help
me? it is very annoying as i dont know if anyone is
talking to me when i leave my computer.
i have windows xp and have updated my nvidia sound
driver.
i can hear a sound when i "switch user" and then when i
switch back, but no other windows sounds.
i can hear music played on my windows media player.
i can hear sounds when i play games on messenger.
i have gone to the options screen in messenger and tested
the alerts and they work there but not when im actually
chatting.
the sounds also work when i test them in the "sounds and
audio devices properties" window.
strangly, when i log on to my brothers account on this
computer, all of his sounds work fine.

im very confused, i have no idea why this has happend. im
not an expert but i understand quite a bit.
someone please help me!

thanks in advance
Daisy