Multiple Instruments playing at the same time

Greetings,
I’ve got the problem that when I open Cubase, I open a project and I try to play on the Keyboard, the playback plays all selected instruments at the same time, i.e. every time I press a button, I hear bass, piano AND drums. When muted, an instrument doesn’t play, yet the solo function completely eliminates all sound.

I assume you don’t mean “All selected instruments” because if you select multiple tracks/instruments Cubase is designed to play them all simultaneously.
Sometimes I get random tracks/instruments playing when they should not be, i.e. I have the Piano track selected and the Oboes are playing as well.
So what I do is:
In the inspector go to ‘All Midi Inputs’, click on it, a drop down menu will appear, click on ‘All Midi Inputs’ again. This fixes it for me. Or sometimes I just click between tracks a few times until Cubase fixes itself. I have also reported this bug in version 9.5.

Any Instrument Track that has its Monitor Button on will play whatever comes into its MIDI Input.

Turn the Monitor Button off for any Tracks you do not want to play via keyboard input.

Thanks, the problem was that the right output wasn’t mapped to anything.