One upon a time in Cubase clicking ‘listen’ on a channel caused the channel in question to be the loudest thing in the mix and attenuated all the others so you could easily with one click focus on any single channel.
These days it behaves the same as solo. The manual says:
The listen mode allows you to quickly check the signal that is coming from selected channels without interrupting and interfering with the actual mix. During a recording session it allows the sound engineer in the control room to attenuate the signal that is coming from one of the musicians while the recording continues undisturbed, for example.
To enable the listen mode, you need to enable the Control Room.
To enable the listen mode, click Listen for a channel.
This routes the channel to the Control Room without interrupting the signal flow.
Have you checked your Listen Dim level in Control Room? If the Listen Dim volume is down at zero it acts like solo. Basically that determines what level the un-Listened tracks drop to. I’m guessing it’s set to zero in your case.
There’s a control for setting the actual Listen level as well should you want that different to your monitoring level (never really understood why anyone would want to change that but still it’s nice to have options!).
p.454 me thinks
5 Listen Dim
Allows you to adjust the volume of the Main Mix when channels are in listen mode. This keeps listen-enabled channels in context with the Main Mix. If the Listen Dim level is set to the minimum value, you only hear the listen-enabled channels.