If we are rightly scared of browser-borne infections and intrusions, then why are we still running browsers on our machines? Why not designate a machine, outside the firewall / in the DMZ, that runs ALL the browsers. The user logs into that machine, and the browser display events are sent back to the client machine. The safe client machine never runs a single snippet of plugin, or gobbles a single byte of untrusted network traffic. The client machine does not even -know- how to get to the internet.
Sending receiving files can be locked down and logged. Or prevented.
The sound device would be a pain, and might require a new protocol, but this would solve many problems. I think it might make SSL better too (no proxy bs).
Perhaps a specialized (corporate) browser nexus product could be offered...with sound and optimized for the browser.
The client machine never talks to the internet. It just sees pictures of it.