Indoor location mapping.

Basically, i'm hoping to set up a sound installation, based upon speakers reacting to location inside a hall. Having looked at this i've been thinking that using GPS may be the best route to go down, but i've heard that indoors this might be a problem? I'm just hoping to map x,y co-ordinates to the volume of the speakers (most likely in Ableton). Any advice would be really helpful.

Depending on how many objects are in the room a few ultrasound sensors may be of help, either reflecting off the object or the object emitting it and timing that to different points that also depends on the size of the room for accuracy