I'm by no means an expert, but this seems to me to be far too subtle a song to start a musical with. The curtains come up, the lights go down on the audience, and you have those nice, quiet piano chords? To be honest, I think it'd be much more effective to smack them over the head with a few massive chords and then drag them into some quieter, nicer bits later on. Like they do in Wicked, for example.
I quite like the track, especially the way the guitar is dropped in like that, though that synth pad seems to eat all the other instruments. And it also gets a bit repetitive, which, again, doesn't happen in so many musicals - each character would, presumably, be changing moods and feelings more quickly than this, necessitating more notes and variation. It does sound as though it's a mysterious, thriller-style thing, and could easily fit somewhere else. But after 3 or 4 minutes, it starts to get old - maybe use the riff as the theme to a character and juxtapose it with a couple of others later on.
Still, for an opening number, I'm sorry to say that the only way I could see something like this working was if the opening was mostly played out in the drama rather than the music, and even then the song would have to build to something, rather than looping the way it does. That's my two cents, anyway.
Reid