Hi, I came across this post that claimed to get the overlay working again (http://espers.net/2013/10/08/mumble-overlay/). I gave it a spin and it actually works for me.
Mumble Version: Mumble Snapshot 1.2.4-178
Windows Version: 8.1
Game Tested on: SWTOR
1. Download the program (http://puu.sh/4tnHs)
2. Unrar and execute the program (run it as admin if the game needs to be run as admin)
3. Run Game
4. Run Mumble
5. Add exception to white list
6. Switch back to black list.
64-bit Windows isn't a problem for the overlay but 64 bit target games won't work natively until we get around adding support for that.
If you found some 3rd party app that makes it work for: nice. Just be aware that running 3rd party tools with administrative (or even user) privileges can be dangerous if you aren't certain about their origins. I don't have a reason to believe what you linked is malicious just though I better mention it.
Yeah its a risk, but I finally got the translucent overlay back.
N.B. Switching modes using the MSI Gaming App (ie Silent to Gaming) does cause the application to crash. It just needs to be restarted. It also looks like the application is an OpenGL implementation that forces overlays to always be on (the MSI Afterburner overlay also switches on when running it, and the crash message mentions OpenGL)
Just a quick note again. I noticed that as of mumble-1.2.4-227-g1bd57bd, I have been able to start Mumble before SWTOR without SWTOR crashing. The overlay appears in SWTOR if Mumble is started first.