It's always a good idea to be using push to talk while playing in lobbies. Your teamates don't want to hear your constant typing and/or breathing while trying to concentrate on doing their job. An easy way to avoid this is enabling push to talk in mumble. Push to talk allows you to talk only when you have a specific key held down. Here's a visual guide on how to enable it:
1. Go to the configure tab on mumble, and select settings.
2. Look for this setting on the main settings screen, make sure it is set to push-to-talk
3. Go to the shortcuts tab of the options, and add a new shortcut
4. Select push-to-talk as the function for the new shortcut
5. Choose a key that you want to activate your push to talk, click on the space under the shortcut section, and hit the key. It should say K:[Key you pressed]
Now just hit apply and ok, and you should be able to use push to talk!