Imac 2019, volume too low

The volume of my iMac Retina 5K, 2019 is really low.

I tried to go into my "system preferences" under "sound", when I select "built-in" the sound comes back a lot louder but only for 1 or 2 seconds, and then becomes real low again...

My Current IOS is BigSur 11.3.1.

What do you suggest?

Thanks. Matt