Remote Desktop Services Remote Audio Issues

You could try and move the Server to use 2008 R2 with RDS. This should allow you to utilize Remote FX, which delivers video much better. You will probably however have to buy some specific cals for your users.

http://technet.microsoft.com/en-us/library/ff817578%28WS.10%29.aspx

Performance issues like you're describing are almost always (in my experience) network related. RDP sessions (in my opinion) are much more sensitive to network issues such as lateny, congestion, and packet loss than they are to almost any other resource issue (memory, CPU, etc).

That being said I've seen enough cases with dumb terminal performance to know that dumb terminals often don't have the horse power to provide a well performing session.

Your own test seems to bear that out; a full fledged computer running Windows 7 doesn't seem to be affected by the problem so that leads me to believe that the problem is with the dumb terminals themselves. If you have a newer, beefier terminal at your disposal I would drop that on the network and see how it performs. I would also try another workstation or laptop to see if you get the same good performance as the Windows 7 workstation. If a more powerful terminal or another workstation exhibit good performance than I think you can definitively point your finger at the terminals.

As a final note, there's only so much tweaking you can do on the server and/or the network before it becomes a "war of attrition" and leads to an excercise in futility. If the terminals are the problem then the resolution is to replace them. I went through this recently with a customer and the final resolution, after all the poking, prodding, and tweaking, was to replace the terminals.