Thanks very much for your input, Gil. I was just thinking if in-wall or in-ceiling speakers can be used. The most ideal setup is to install speakers on each floor in the shaft so when the elevator car (also made of glass) travels through the glass shaft, sounds would drift in and out of earshot. I am aware of the possibility of a lot of attenuation of sound since the the car is enclosed in the glass. If the speakers are placed in the car, this would be become a different installation piece, conceptually.
I am thinking of using at least 150-Watts speakers to connect with an amplifier - do you think it would suffice?
Thanks again!