Yes, sounds familiar. Audio doesnt work but video does.
If its not too much trouble (and only if you wish to be helped

) can you copy and paste the error message and describe the process?
For instance, when I had my audio problem, after my contact accepted the invitation the message ''connecting'' would show up on the bottom of the MSN chat window. This connecting process would last for about 10 or 15 seconds and then would be terminated, probably timed out and an error message like this would come up: The computer or network you're trying to connect with is unavailable (unavailable or busy, cant recall), please try again later.
From what you've said so far it sounds like an RTCDLL.dll file problem.
Did you use to have MSN 5 that worked?
Did audio NEVER used to work?
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