You still have to look somewhere. Lots of people certainly do listen to music, judging by all the people wearing wireless earbuds I see, but if they're not staring at their phone, they're staring at their lap, or sleeping.

It's not quite this bad on uncrowded trains though. We're talking about the Yamanote line which is generally very crowded.

Do it with your eyes closed or stare blankly at the advertisements that plaster the walls of the train (conveniently located high up at the ceiling).