I got hold of an original HTC Desire 310 battery. It seems to work just fine, but when I ran the speaker test in the CSD app, it produced a sound for about 0,5s, and then silence. Reboot, power cycle, nothing mattered, not a single beep was produced by the speaker. I really though the speaker was now broken.

Well, you've guessed it, swapping the battery for the original Jolla, and voila, the speaker works perfectly again. Swapping again, with the HTC battery: no sound.

WHY? HOW?

No seriously, what can cause this? Even with a charger connected it doesn't work, so it doesn't seem to be a shortage of available power. (The battery isn't new)