im not 100% of how the sounds from avast get addressed, but i guess they get addressed using a windows subsystem.
Since there is no soundcard or driver installed for this on the ex470 avast cannot play the sound. With accessing the server from a client i assume here that you use the Remote Desktop Client that is buildin in windows xp/vista/7. Since the sound subsystem on the server is not running it can not send sounds to you clients since the subsystem is not running to send the sound...
now i hear you think: but i have itunes streaming wonderful.... and there is the catch, the streaming works differently it doesnt need the windows subsystem for streaming the sound since it is working in a different way
if you want to uninstall avast (even tho the reinstall wont have the effect that you want, i think) you can download the avast uninstall tool
http://www.avast.com/eng/avast-uninstall-utility.html, that should work to remove avast propperly