Mumble spawns a zombie ("defunct") process if it can't find sys-apps/lsb-release installed. Per upstream: http://sourceforge.net/tracker/?func=detail&atid=768005&aid=2960928&group_id=147372 Please include sys-apps/lsb-release in media-sound/mumble's RDEPEND.
Huge thanks for investigating the issue and taking it upstream, fixed in >=mumble-1.2.3_rc1.