Choose what you hear
Two taps. One tile has the sound and the rest stay quiet, so the wall never turns into noise.
TV A
From your phone
In Monitors, tap the tile to open its sheet, then tap the speaker button in the row of small buttons under the playback controls. Every other tile goes quiet.
You can tell it worked three ways: the speaker button stays pressed while that tile has the sound, the tile gets a small light in its corner in Monitors, and on the TV it gets a thin white border.
If a tile is showing a web page rather than a service, its sheet has no speaker button, so give it the sound with the remote instead.
From the remote
Press an arrow to bring up the highlight, move it to the game you want, and press OK. This works on every tile, web pages included. (The remote works on a Mac only for now.)
Volume and mute
The volume slider and Mute on the phone’s now-playing bar, and the remote’s volume and mute buttons, all change the computer’s own volume. If the sound goes to the TV over HDMI, the computer can’t change it, so they do nothing: use the TV’s remote to turn it down or mute it.