The old news about Mumble were - tricky licensing terms stopped the work, but there were rumors for Qt (heavily used in Mumble) to change license to LGPL.
Not long ago, the rumors came true and integration is now possible.
If you are really interested in it, you can help, telling how do you think such integration should work/look like.
It may even be possible for DCS to provide script (lua) hooks which will help to integrate ANY VoIP solution.
Please, share you thoughts.
P.S. Still, I (personally) think that ideally, DCS should have an internal voip (which will simulate aircraft communication system).
But so much to code, so little time.