Let me clear up any confusion about what our plans are for audio APIs in Firefox.
Some MediaStream support has landed in Firefox 17. We have the ability to create MediaStreams containing the output of media elements and use them as a source for other media elements. I need to make some changes to those APIs based on feedback we start evangelizing them for real. We can also create MediaStreams via getUserMedia (when that feature is preffed on in about:config).
The work on MediaStreams Processing that I did as an alternative to the Web Audio API is on the back-burner. Our priority is to implement Web Audio. Our plan is to implement AudioNodes using the same infrastructure as MediaStreams under the hood --- to reduce code duplication and to ensure that Web Audio/MediaStreams integration is perfect. Some core infrastructure for MediaStreams that are produced by processing inputs to outputs --- ProcessedMediaStreams --- already landed, to support the features above. Currently Ehsan is working on the IDL/DOM API side and I have some more work to do on the MediaStreams infrastructure side. We don't have a specific date set for Web Audio support, but it is a high priority.
At some point we will revisit MediaStreams Processing to get the features that Web Audio is missing, e.g., seamless stitching together of an audio and video playlist from a series of clips. That is lower priority.