If you've ever wondered what it's like to be working as a musician on such a show, here's a little 3 minutes behind the scene which gives a pretty good idea. 
One of my favorite Cirque shows. 
 

 

Very cool. Very professional.

Fascinating. I am always in awe of those kinds of real-time productions.
 
The last time I saw a show like that, they had live musicians on stage but it was obvious that a large part of the soundtrack was also pre-recorded.  I'd heard that there are union rules about how much of the music has to be live versus recorded. I don't know how true that is, but it makes sense - Vegas is a union stronghold.

A lot of the shows have musicians on stage, usually in some kind of nook somewhere. For example, in Zarkana they have those two structures, one on each side of the stage...

 
In a lot of shows, singers and sometimes musicians will occasionally make an appearance onstage at some point. Which is why they still dress up and put the make up on.
 
The guitar player in KA (who happens to be a friend and used to play for my wife back in the days) has his featured spot during the show.
 
For my wife's show, I know that they are using Ableton Live! to launch some tracks and cues, even though the musicians and singers are often multi-instrumentalists and play the majority of the music. 
 
The funny thing is that I'm waiting for my acoustic guitar to arrive to complete the audition kit and at least one of those songs I have to learn is from KA and the reference track features our pal. Another one features the male vocalist who used to be on the same show as my wife - a dear friend, really.  It was an odd moment when I heard his voice through the monitors in my home studio...