There doesn't seem to be a way to turn off sound in individual videos when layering a composit video. Often I would want to turn these off to be replaced by a master sound track. Is this an oversight?
You can mute individual clips on your timeline by accessing the audio options flyout. Either right/ctrl + click on the selected clip area, or click on the flyout triangle at the top right corner of the selected clip, then click on the music note button. Check on the Mute Audio check box to mute the clip.
For smart object clips, simply open up the layer/clip as a smart object, follow the same instructions above, save, and then close to return to the master project.
It's true that with our design you have to mute individual clips and there is not a function to mute entire video tracks. This is available for audio tracks, however.
Hope this helps!
If your video is in the form of a video clip, just click the fly-out triangle at the end of the blue clip.
Click on the little music notes in the resulting menu, then check Mute Audio.
Add an audio track using the + button at the end of the Audio Track, or use the Add Audio command under the music note button in the Audio Track.
If your video is in smart object form (it'll be a purple track, instead of blue), then you'll need to edit the smart object to get to the video to turn it off.
Does this help?
There are disadvantages and advantages to nesting Video in a Smart Object.