But really, ALSA itself works fine. Every test reveals that.
It is all and only the applications that depend on gstreamer that are
silent. I thought that those applications might be directing sound to
the crappy onboard card by default, but I checked, and that is not the
case. I ran gstream-properties and explicitly selected Alsa as the
output plugin, and the default card (card0) as the default device. Checked
the pipeline in gstream-properties audio output and it produced the
tone with no difficulty and sent it through card0. But it remains true
that those applications that depend on gstreamer for sound output will
not send audio-data to that card.
So it's some problem either in gstreamer itself or in its negotiation
with Alsa, a problem which didn't exist before the upgrade.
Time for a bug report, I suppose,
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