Noticed this while working on re-enabling tests that were disabled due to bug 1714410. It seems that this test has completely stopped working sometime since it was disabled in april.

https://treeherder.mozilla.org/jobs?repo=try&revision=f218be1fb37b10c2b710c5fb27b23bdc181895ef

navigator.mediaDevices.enumerateDevices is just not returning anything of kind 'audiooutput'

Thanks for filing.
An audiooutput device needs to be exposed before it will show up in enumerateDevices() output.
Moving enumerateDevices() to after getUserMedia() would be sufficient on Linux test machines, but selectAudioOutput() might mean the test can run on other platforms also.
e.g. https://hg.mozilla.org/integration/autoland/rev/35d3a828f598#l2.12

I don't know how the opt tests can be passing.
https://searchfox.org/mozilla-central/rev/d8d7539593d5a3071841289dee72f971650073ce/dom/media/webrtc/tests/mochitests/mochitest.ini#69
Nor why https://hg.mozilla.org/try/rev/5b2572b7e09a27ea8f7f122ff67d8a91b9ae3a27 would change opt results.

(In reply to Karl Tomlinson (:karlt) from comment #1)

I don't know how the opt tests can be passing.
https://searchfox.org/mozilla-central/rev/d8d7539593d5a3071841289dee72f971650073ce/dom/media/webrtc/tests/mochitests/mochitest.ini#69
Nor why https://hg.mozilla.org/try/rev/5b2572b7e09a27ea8f7f122ff67d8a91b9ae3a27 would change opt results.

Ah, os = "linux" is not a python (equality) expression.
https://hg.mozilla.org/try/rev/5b2572b7e09a27ea8f7f122ff67d8a91b9ae3a27#l1.35

The Bugbug bot thinks this bug is a defect, but please change it back in case of error.

This was disabled for all builds due to a failed attempt at equality syntax.
https://hg.mozilla.org/try/rev/5b2572b7e09a27ea8f7f122ff67d8a91b9ae3a27#l1.35

"audiooutput" devices are now not exposed before permission is granted.
https://bugzilla.mozilla.org/show_bug.cgi?id=1493982

Depends on D126630

https://hg.mozilla.org/mozilla-central/rev/024b338382da
https://hg.mozilla.org/mozilla-central/rev/4e777a704dee