Skype is proprietary software that allows you to make calls over the Internet using your computer. Skype uses semi-decentralized peer-to-peer technologies, so your calls do not go through a central server, but through a pool of dedicated servers. It uses its own proprietary communication protocol to achieve this. In addition, all communications are encrypted from end to end so that others cannot listen in. The Skype software is free to use, but it is not free software; the source code is proprietary and not available for review or modification The latest release Skype 4.
Mumble is a low-latency, high quality voice chat program for gaming. It features noise suppression, automatic gain control and low latency audio with support for multiple audio standards. Mumble includes an in-game overlay compatible with most open-source and commercial 3D applications. Mumble is just a client and uses a non-standard protocol. You will need a dedicated server to talk to other users. Server functionality is provided by the package “mumble-server“. This new release contains two client-side security fixes, you should be fine if you are still running 1.