My BM235 suddenly got very quiet today, mid-use.
I first thought about disassembling it to replace the buzzer but that seemed way to complicated...
Since the buzzer was broken anyway and would have had to be replaced I thought about opening it first to have a look inside.
There is a small gap at the side that allows to easily remove the buzzers top cover with a small flat-head screwdriver or a knife.
Inside is the actual piezo.. there was an easily visible layer of some kind of dirt on it.
The dirt was covering parts of the crystal and was thicker in some regions (unfortunately I didn't take any photos).
Using some paper towels and a bit of rubbing alcohol it was very easy to remove.
Afterwards I placed the piezo back into it's housing (smooth side up) and closed the meter.
Before closing the buzzer I also bend the springs a bit up for better pressure (probably not necessary).
After closing everything back up the piezo now sounds as loud as it probably should.
I guess this is some kind of manufacturing defect and the buzzer just moved slightly and didn't make good contact anymore. I'd guess that other meters have a similar issue. So before taking out the soldering iron maybe check if my solution also works for you