#1
Go to the Control Panel >> Sound select your TV then click Properties, check that "Device Usage:" is enabled.
#2
If it is enabled and still not working, then in Sound select your computers audio devise, click properties and diable it.
It you have to do the second, then after your done using the tv you'll have to re-enable your computers adio device. You will have to go through this disable/enable process every time.
Hope this helps