Firewall preventing Airplay for non-admins

Hi,

I've got an Airport Express connected to my hi-fi and play music from iTunes through it. For me it works perfectly, but for the other accounts on the Mac, when they try to connect to the hi-fi in iTunes (using the Airplay button) they are told that the firewall is blocking Airplay. They are also asked for an admin user before anything can be changed. I found that iTunes was being blocked so set it to accept incoming connections. All seemed okay.......

Except - the next time you start up iTunes the problem is back and the firewall setting has not "stuck", so it all has to be done again. This is really annoying and prevents anyone in the house streaming music to the hi-fi unless I am there with my admin password.

Does anybody else have this problem? Why won;t the firewall changes "stick"?

Thanks in advance,
Chris