So, the speaker issue is definitely improving.
With the battery fully charged (and /sys/class/power_supply/battery/voltage_now >~ 4100000 ยตV), the tapping sound reappeared sometimes while changing the volume of a playing youtube video, though not to the extend to make the video unwatchable.
Now, with the battery charge at 72% (/sys/class/power_supply/battery/voltage_now ~ 4000000 ยตV), I'm back to "cannot reproduce" ...
I am now quite confident that this problem will quickly go away with ageing of the battery, as others have observed, and I regret opening the Pro1-X for this (