Cactus Music
And some one had this problem the other day and found they had hidden tracks from some out takes. But your was a simple signal path issue.
I had a similar weirdness and turned out Sonar had somehow created an extra copy of the Master buss and it was off screen.. It was even weirder because one was master and one was Master.
Yeah I still have no explanation for when everything was muted and still all were audible. I looked for hidden tracks. And I don't have multiple takes so.... I dunno