There are no such things as "stupid" questions. However if you think your question is a bit stupid, then this is the right place for you to post it. Please stick to easy to-the-point questions that you feel people can answer fast. For long and complicated questions prefer the other forums within the support section. Before you post please read how to get help
Hi! Install it via Synaptic Package Manager. In the menu: Administration > Synaptic Package Manager. The new window shows up, click on "Search" and write "Mumble", then you find the mumble. Right click on it and choose "Mark for installation".