There may be a new cool simple way or button to take care of this, but if I understand your post correctly...did you try option+selecting just the video tracks only and then right-clicking and selecting "nest" then right-click again and select multi-camera>enable. That is how I used to do it on CS6 and it worked recently on CC. That way your video is nested all on one track but your audio remains untouched. I don't know why when you did it some of the tracks turned off. Might be something simple that's not being toggled on or off, not sure. But you can try what I suggested for now and see if it helps.
HOWEVER...please be aware before you start your multicam edit that there is a huge, giant, ugly, ridiculously unacceptable bug with multicam in Premiere Pro CC right now that has essentially broken the multicam experience. If you don't know about it, read about it below or you may be left losing hours of editing work:
BRILLIANT! I never knew I could do that! I've always add the individual audio tracks to the multicam after I made it. This is **way** easier.
now if only I understood Spanish
Oh... I do know about the multicam problem. It doesn't really effect the way I work since I only make the roughest of rough cuts "live". Almost everything I do is in the timeline but I can certainly see how it's a deal breaker for some people