new audio layer on project - no sound!

I have an project saved which I started in June, and now going back to it. There’s just one Instrument (midi) track for my midi-keyboard (with vst effects - setup made). Now I wanted to add Mono Audio layer and play the synth together with the midi -but for some strange reason I cannot get any sound out from that synth to Cubase! I have the monitor button on, my soundcard clearly shows that the sound goes in - but Cubase doesn’t catch it.
If I start a new project with Mono Audio, with that same synth - there is a sound! So what could be wrong on that June-project file, why the sound doesn’t come in to that project from my synth???