Yes, when playing midi files it shows activity.
Just to be clear, I’m sure you know the difference between audio activity & midi activity in Cubase. Midi activity will be in the Transport bar meter. You should see audio in your mixer view.
To be able to hear audio in Cubase… in the Arrange Windows track lanes …do you have corresponding audio channels? Are they set correctly? You should see at the left side of the Arrange Window when a track/lane is highlighted, a selection of the intending I/O from an audio device. Are they enabled & are you monitoring them correctly?
If I connect headphones to the MOXF8, I hear the music, and the volume control slider on Cubase functions. The only issue here is after 5 to 10 minutes of play I start getting crackles, clicks, pops, … from the sound heard on the head phones.
Do you get these crackles & pops only when the USB is used, or do you get it when nothings plugged into your keyboard except your headphones? In other words, does it only happen when interfaced with Cubase? If it happens with your keyboard only with headphones and nothing else, I’d worry about the keyboard.
Buffers/latency settings? Samples rates matching up between your equipment & Cubase?