There are not too many people using mumble, mostly us admins and a few other people. We play on TDM most of the time, but when there are enough people, we decide to go to the bomb server most of the time. Yet we are still using the same channel we were in, when we played on the TDM (there is no autoswitch enabled for the bomb server as there are so little people playing on there anyhow). If other people wish to actually use mumble for TDM while we are playing bomb, it is no problem. We will then switch channels if asked so. But the thing is, aside of us and a few regular people we know, nobody ever uses mumble despite our efforts and advertising
So it is only expected behaviour that you heard the people from the bomb server as we occupy the same mumble server and (most of the time) the same channel as on TDM.