As of today (Jan,15 2012) the digital signature of mumble version 22.214.171.124 is
no longer valid .
"Certificate valid from 14-1-2010 till 15-1-2012"
hopefully the owner will be able to provide an update asap
a workaround that should be working be lowering the date of your system back
to jan,14 2012.
once its launched you can readjust your date