CEF does not appear to support routing of audio, there is a PR to add it but it is stale and not near being merged: https://bitbucket.org/chromiumembedded/cef/pull-requests/18/adding-audio-support/diff
This means that any audio played in it gets sent to the system default soundcard, rather than any audio consumer for the channel. It also means that any audio mixer commands will have no effect.

Once CEF supports routing of audio we should update our html producer to make use of it.