I'm guessing you are struggling with the auto echo of the selected track. Click the input echo button until the a goes away.
This still confused me too sometimes. Midi works while the track is selected... Click on another track and it goes mute.
If you are a midi musician, you often want to record a single midi input at a time. The track with the focus will be the only one you hear at any given moment.
If you have a bunch of midi controllers, this is confusing behavior.