This is the answer of pi-top support:
As mentioned previously, we don't officially support ubuntu-mate.
So we cannot reference how to get the speaker working.
Once again I do apologise for any inconvenience caused.
All the best,
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On Tue, 17 Jan at 8:32 PM , robinie <firstname.lastname@example.org> wrote:
with pi-top OS the speaker is working.
So I know the speaker itself is o.k. / has no damage.
But for my purpose I need another OS. Ubuntu works, but without audio.