I’ve been trying to use the “audio conversation” when chatting with my friends in US on msn, but each time I want to start it I get a message telling me that I am in call and the audio C stops.
What shall I do?

Try shutting the whole msn and opening it again.

Otherwise i'd recommend that you use "skype". Its a voice conversation program which gives just what msn does.

Read about it in this thread (http://www.englishsabla.com/forum/showthread.php?t=22863)

Thank You Evilio :)

No problem :)
Hope it worked for you.