Have recently upgraded to Mumble 1.3 on devices I use to communicate with RigPi. Tried the suggestion in this post (after making a backup), and had the following result:
pi@rigpi:~ $ sudo apt-get install mumble
Reading package lists... Done
Building dependency tree
Reading state information... Done
mumble is already the newest version (1.2.18-1+deb9u1).
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
So it looks like the apt-get package has not been updated to 1.3. Had no problems with the 1.3 upgrade on Windows or OS X. Those clients work the same with RigPi, but have a cleaner interface and the minimize button works, which is a big plus.
'73 - AE0AU