I worked around the problem, without really solving it.
I tried copying and pasting the problematic track to a new track. Result: the same thing. Silence.
Then I copied different kinds of messages one at a time, those that I knew I had put there on purpose. Notes, pitch bends, and volume. And it worked!
I can hear the track normally.
I looked carefully at the event list, and couldn't find other messages at all, not to mention something that might have caused the problem.
So, it's still a mystery, but it at least I can get on with my work.
Still curious if anyone has an idea what might have been causing the problem.