you dont need mumble to play but it is highly encuraged and makes the game much much funner.
Anyway...
HOW TO GET INTO MUMBLE FOR ALL THE NEWBIES
1. Start Mumble with desktop icon ("PR Mumble")
2. Press "Configure" select "Audio Wizard"
3. Follow the on screen instructions to configure your audio settings
4. When complete Press "Configure" and select "Settings"
5. Click the "User Interface" Tab and check the box "Expert Config" then click "Apply"
6. Click the "Audio Output" Tab and be sure you have the following, then click "Apply"
Volume -> 60%
Other Applications -> 100%
Bloom -> 150%
(These should be default)
7. Click the "Shortcuts" Tab, edit the Shortcut for "Local Speech" to be the button that will talk to people near you. (I use middle mouse; it doesn't matter what you use as long as its not already assigned to something else in game)
8. Click "Add". In this new box under select "Squad Leader Radio" for function and select a shortcut button for it (just like you did above). This button is less common then "Local Speech" and is used to talk to other squad leaders in a server. (I use "\" for this)
9. Click "Apply" and close the Mumble Configuration window.
10. Press "Server" and select "Connect" in the top menu. You should see the Project Reality Official Mumble server. Type a name in for "Username" and click "New" this is the only server you will need to connect to.
11. Click "Connect" and Click a channel in the server applicable to the server you wish to play in once inside PR.
12. In the channel you may also have to join the sub channel for the proper team (Depending on server) or the squad leader channel for your team if you wish to squad lead. Often times admins will do this step for you if you ask...
Enjoy
ps If anyone sees any mistakes in here let me know and I'll edit the post.