Problem with alert sound

I have got a weird behavior of the alert sound in ubuntu 16.04.2 LTS + Mate Desktop. If I have multiple terminal windows open, the sound is very weak in most windows, zero in some, and sometimes quite loud in another. It does NOT at all react to the alert volume slider in the sound preferences window. When I close this window and open it again the alert volume slider jumps to zero. If I get alert sound at all, it DOES react to the Output volume slider.

Otherwise sound works fine, e.g. with rythmbox.

Any ideas?

ulrich