SILK, the codec behind the truly and justified-successful VoIP-software Skype is now used in Steam. Let me quote what Valve said about it.

Steam's voice chat system now leverages the SILK audio codec, developed and used by Skype, makers of the world's most popular voice communication service. The SILK codec provides a significant quality improvement over Steam's previous voice technology, at the cost of some increase in bandwidth usage. Steam Voice used to require 15 kbps of bandwidth, whereas SILK is a dynamic bit rate protocol which varies in its use of bandwidth between 8 and 30 kbps, depending on the range of data in the voice signal and current network conditions.

Read the full story @ http://store.steampowered.com/news/5100/