Multiple instances of the Mumble client I have tried command that I found and it works for second and after that second client closes.

open -na /Applications/Mumble.app


Other command that I tryied with the same resul.

/Applications/Mumble.app/Contents/MacOS/Mumble &amp;
[2] 27307
[1]   Exit 126                /Applications/Mumble.app


What command I must use to have multiple Mumble instances open?
What you and future searchers are looking for is this:

open -n /Applications/Mumble.app --args -m


This is valid as of 11/04/14 on OS X 10.10/Darwin 14.0.0 (Yosemite)