Is there a quick fix to enabling audio within the unsafe web browser? (E.g., HTML5 audio test page http://www.terrillthompson.com/tests/html5-audio.html.)

Is there a quick fix to enabling audio within the unsafe web browser? (E.g., HTML5 audio test page http://www.terrillthompson.com/tests/html5-audio.html.)
Beware: the Unsafe Browser is not shipped in Tails for general web browsing usage.
Regarding the actual question: I suggest you dig on the PulseAudio side.
(Other Tails developer here)
I think a valid use case for sound in the Unsafe Browser is to make it usable for people with sight impairments. I'm unsure whether Tails includes any text-to-speech software that actually makes Tails usable with such impairments, but even if it doesn't the support should be there for the day such software is added. Hence commit f6dba62.
Actually Tails doesn't use PulseAudio. The fix is to add the
clearnetuser to theaudiogroup. Note that doing this in a running Tails session isn't as easy as one might think since the Unsafe Browser runs in a chroot based on the filesystem on the read-only media; any changes made in the running Tails session (e.g. running the expectedsudo adduser clearnet audio) won't be visible in that chroot.