How do I listen with the picture off?

Under the video, tap Audio only. The picture goes and the sound stays. Tap Video to bring the picture back.

You can also start a video with sound only. In its menu, tap Play audio only. See The video menu.

To start every new playback with the picture off, tap Settings, then Playback, then turn on Audio only by default.

Audio only is part of Pro. On the free plan, it opens a panel that says what Pro adds. See Background play to keep the sound going with the screen off.