Transcoding audio with MediaRecorder

I have been experimenting with Firefox's Web Audio (decoding) and
MediaRecorder (encoding) APIs to transcode audio between formats, such as
transcoding a .wav file to Opus in the browser. A file can be decoded with
the Web Audio context's decodeAudioData, and the resulting buffer connected
to a media stream destination node, whose stream can be recorded with
MediaRecorder.

The big problem is this can only transcode audio in real-time. For a tool
based on this to be useful, it should be able to transcode as fast as it
can. This suggests the use of an OfflineAudioContext, but these cannot
create media stream destination nodes (at least in Firefox, when I tried).
It's not clear to me, but I get the impression a MediaStream is a real-time
stream and cannot go faster-than-realtime anyway.

Does there need to be a new API to enable this? For example one solution
could be to pass a Web Audio AudioBuffer directly to MediaRecorder instead
of a MediaStream. AudioBuffers provide a sample rate, channel count, length
and contain audio data, which seems to be everything MediaRecorder needs to
know for fast-as-possible encoding.

One other problem I had which may simply be a Firefox bug is that starting
the MediaRecorder and audio buffer source at the same time with code like
this:

mediaRecorder.start();
bufferSource.start();

...appeared to skip the very beginning of the audio buffer. I guess
MediaRecorder takes a moment to start actually capturing audio, and I
should start the buffer source in mediaRecorder.onstart, but that function
was not firing in Firefox. (Not yet implemented?) For transcoding it is
important that the MediaRecorder can capture the exact same number of
samples as are present in the original audio buffer.

Ashley Gullen
Scirra.com