probably best summed up with - "use some other sowftware !" - till MS sort it out
MSN Messenger has been having all sort of problems since about ver 6 - with one end not being able to hear (or see the other end)
Refer
HERE for more detailed help/suggestions (EDIT: changed link)
I now use EyeBallChat (since I use a camera too, think it works without too) but the sound quality is much better that MSN, and now theres SKYPE too to consider