Intriguing sound problem under Mojave

Hello, I've just installed a fresh copy of Mojave.

The problem is that the sound up and down keys (F11, F12) do not provide any sound feed back, although the volume slider moves left and right. Same problem with the Sound panel of the system preferences : no sound feed back when the volume slider is moved.

However, all the apps that produce sound seem to be ok : the sound is output either on the internal speaker or on the monitor (Cinema Display) be it through the USB cable or the video cable. Likewise, mail notifications do produce the usual beep.

I have tried an other install but still the same issue.

I have Mojave installed on 2 other drives and no such issue.

I am reluctant to completely erase the volume and undertake an clean install.

Any help is welcome