I am very wary of the "kick their arses into gear" flavor of this.
First of all, it just doesn't seem like a friendly or positive attitude for the producers or whoever asked this of you. Are difficulties expected? Are they being overly demanding before rehearsals even begin? Do they know something you don't? When you expect the worst, you will always get exactly what you expected.
Frankly, you're working with a group of professionals. You don't need to explain that they should do a great job, because their job is to do a great job. It's not like they were expecting to come to work and completely brush this one off, until your rousing speech convinced them otherwise. And when someone perceives that the actors are not giving it their all (or worse - the show is not making enough draw), will the producers ask you to take it up another notch? When will this stop being your responsibility (and why is it your responsibility to begin with?). I'm just a little jaded and I hope my worries for you are totally unfounded.
But you asked about welcome speeches, which are totally standard and kinda fun and always a good welcome event. So...
Be positive, be enthusiastic, be real. We have really great feelings about this show and the wonderful cast and crew working together on it. We all need to cooperate and communicate to make the show as successful as it can be. The SM is here to help - talk to me anytime and I will do everything I can to keep everone comfortable, creative, safe, and happy. We are taking a fresh new approach to this show, and it will be a big and complicated production, but exciting. We'll work hard, and it will be a great show. Here's your director. Here is the set taped out, there are the bathrooms.
Scene 11, please...