Make sure QoS is off too.
What are the bandwidth requirements?
From 0.9.1, this is highly variable, and mostly up to the user. With top quality, minimum latency and positional information sent, it is 144.0 kbit/s including the IP and UDP overhead. With 60 ms transmission delay, the lowest quality speech and no positional information, it is 15.8 kbit/s (again with IP and UDP overhead). The default quality setting uses 58.8 kbit/s. When comparing with other products, remember to compare the total bandwidth use and not just the bitrate of the audio encoding.
There are two parts to tuning the bandwidth; the audio bitrate per audio frame (e.g. 10ms) and the amount of frames to put in each packet. Each transmitted packet has an overhead of 28 bytes from IP and UDP alone, so at the highest transmission rate (100 packets per second), that is 2800 bytes of data for raw network overhead alone. You should try to find a balance that works well for you, but we generally recommend sacrificing high audio bitrate for lower latency; Mumble sounds quite good even on the lowest quality setting.
There is no way to adjust the amount of incoming bandwidth; you will have to have enough to sustain the total amount of speaking players. This should be a minor issue; most players these days are on asymmetric lines and hence it is only upload that is a bottleneck.
Is Mumble encrypted?
Your whole communication to and from the server is always encrypted. This encryption is mandatory and cannot be disabled. The so-called control channel, which transports your chat messages and other non-time critical information, is encrypted with TLS using 256 bit AES-SHA. The voice channel carrying speech and positional audio is encrypted with OCB-AES 128 bit. You and the server authenticate to each other using digital certificates like they are used for secured connections in Web-browsers.