Got it.
I measured the crystal right at the beginning of the whole troubleshooting and it did put out the correct frequency.
With everything else basically ruled out (all traces checked out fine and most of the involved ICs swapped with known good ones) I started heating and cooling down parts on the board to see if something changes...
and so it did. When heating up the crystal right next to the ASC the sound came on. When letting it sit to cool down sound disappeared again.
It must have been putting out a weak signal - heating up makes the crystal swing more which strengthens the output signal.
Replaced the crystal resulted in perfectly working sound output.
Another case closed