Make sure that only the event that you want to make a change to is the only event on the timeline that is highlighted (with a yellow frame). If more than one event is highlighted, then changes such as muting or adjusting opacity or audio level will affect all the highlighted events no matter what track that they are on.

I just loaded 2 video and two audio tracks. I grouped them and checked by moving one and they all moved. I ungrouped them one at a time and moved them one at a time to make sure they were ungrouped. Then I selected an audio track. The track I selected was the only one with a yellow frame. I then clicked on the M in the header of the selected track and the M in the other headers also were highlighted. The preview screen went dark and I could not play any sound.

So, I went Edit-Group to see if there were any selections available. All of the selections were grayed-out. I could not make any selections.

@Don-Theyken 

• The video & audio on the tracks are called Events. ('one with a yellow frame')
• Events can be grouped together but tracks can be grouped also.
• When all tracks are selected the header is only a slightly different grey to not selected....
• Click on the grey track header to select just one track,

I then clicked on the M in the header of the selected track and the M in the other headers also were highlighted

@Don-Theyken 

Edit - Select all or Ctrl+A selects all tracks & events, maybe you did this inadvertently at some point & selected all tracks. All tracks will stay selected even when you click around on the events on the timeline, click on the tracks of the timeline or when you click the M button. These tracks only become unselected/individually selected if you click on one of the track headers grey area beside the sliders or the FX, M, S buttons. .

Hi,

Thanks to all for the help. What I found is, to break the connection between all of the tracks is, remove all the tracks from the group, then click on the colored part of the header and then all works normally. Click on the M one track and only that track mutes. Back to work.

@Don-Theyken That's what @DMT3 & I wrote.

You had all tracks selected not grouped, you can't just click on a track to ungroup it from a group of tracks, but you can select just one track that is in a group of tracks.

To ungroup a track from a group of tracks you have to right click -