Are there any "thin clients" that would allow me to run a Windows XP/7 app on an iPad with sound?
The only rdp client I know of that records through the iPad onto the host is "iRDP AV".  The only problem is it doesn't have the functionality that many other RDP apps have such as two-finger scrolling, etc.  It's only 2.99 though and definitely worth it if you just need basic desktop control and mic functionality.