I have this problem as well and just now came to the forum to see if anyone else has it.
It started recently after no changes. (More precisely, I took a chance and updated from 8.010 to 8.035 when 8.035 was released, but this problem occurred just in the last few days.) Randomly there is no sound from midi tracks. Sometimes it is many tracks at once (once including almost the entire project - all synths and GA4 drums run by midi), sometimes only one track. The last time several tracks’ sound disappeared like this, restarting Cubase several times somehow made the problem go away. Right now I have two Serum instrument tracks making no sound although strangely the spectrum analyzer is jumping up and down in the channel’s EQ as if there is sound. But the channel’s level meter in shows no signal.
Update: I just restarted Cubase and now it is three Serum tracks making no sound.
Update: I just restarted Cubase for the third time and the sounds have come back.
I hope this information helps sort out problems. I won’t be able to check this thread again.