A hardware based system is going to go do what
you want it to go. You will have to get a program
like CyBlock, which includes "Streaming Media"
in the category list. CyBlock, while it is very
expensive ($799 per year for up to 10 machines),
it is the only solution that will do what you want.
The "Streaming Media" catergory will block
streaming audio you do not want on your
network. Just do that, and then configure your
firewall so that only outgoing calls to ports
80 and 443 are permitted, that will force
any media players installed to use the
HTTP-only option, forcing everything
through the filtering proxy. You will just
have to convince your boss to spend
the big bucks for a license, plus a Windows
based server (if you do not already have one).