Internal speakers not working despite showing as output device

Dear all,

I would like to make use of the vast knowlegde of this user base, so here it comes! The following problem occured: I received an iMac from work that was being replaced, thus I do not know the status of the iMac, although it was not replaced because broken, just as an update.

I opened the device via removing the screen removing, to clean it form dust and to check if a SDD or HDD was installed. Fortunately an SDD was installed, and not much dust was gathered internally. So I cleaned the vents and the rest carefully with a duster. Afterwards I installed MacOs, which succeeded using internet recovery.

Then the problem starts to occur: only the left speaker was working (I don’t know if the right one was already broken), and a few days later the left one stopped working. Also the microphone seems to be broken.

In MIDI setup everything seems normal, I tried every setting. In sound board in settings the internal speakers are showing and volume can be changed as if nothing was wrong. Furthermore, sound works with external headphones plugged in, or using airplay. So the DAC should be working, and probably the audio card too. Maybe the amplifier for the internal speakers, or the internal speakers are broken?

I tried:
resetting PRAM, SMC, doing the apple hardware diagnostics short and extensive, killing AirplayXPCHelper, killing coreaudiod and checking the cables from logic board to speakers, which i disconnected and reconnected. Now I am stuck.

Does anyone have an idea? E.g. how to test if the speakers are functioning, or how to test if the audiocard is functioning? Or where on the logic board is the audio card connected? Every info is helpful!

Thanks in advance,
Remi