Mozilla's Opus codec

Opus audio codec is supported since Firefox 15, and is being introduced as the codec which provides the highest quality and lowest latency with the same bitrate as of today.

Google doesn't supports Opus in their Chrome browser yet, however, they proposed this codec as the standard audio codec for WebRTC, so the future Chrome with their own Blink engine may support this one.
http://www.ietf.org/mail-archive/web/rtcweb/current/msg04953.html