Avast needs to do something about this conflict between the Firewall component and Air Play. I’ve just upgraded to Windows 10 and found that I can’t turn on Windows Defender Firewall while Avast Antivirus is installed, despite the fact that I’ve turned off the Firewall component. So there’s the choice of using Airplay or having the Avast Antivirus Firewall turned on - not both at the same time.