Power can certainly be an issue, especially when the stereo system is sharing
the same power line with the air-conditioning machine. However, changes in
the density and/or pressure of the air can also be responsible for what you
hear. To put things in perspective I would note that:
1) Our ear drums detect sound by detecting variation in the pressure of the air
molecules.
2) The drivers of the speakers are pistons that move the air in the room. They
create the variation in the air pressure.
3) The speed of a wave (sound wave in this case) in a medium depends on the
pressure and density of that medium (the air in this case). Cable manufactures
often talk about this phenomenon.
Thus, it should be clear that the humidity and pressure of the air in the room
can affect how well the waves emitted by the speakers will be transmitted to
our ears. The air in the room can be view as a cable transmitting the signal
generated by the speakers to our ears.
I do not know the actual magnitude of all these effects (they all are very
small), and, for sure, there are more factors that play a role (e.g. ionization
state of the molecules). However, since our ears are extremely sensitive
instruments, I would not be surprised if people (or at least some of them) can
hear the effects induced by these perturbations.
More significant and trivial effects are the currents of air and the noise
created by the air-conditioning machine in your room. But I assume you are
not referring to those.
Of course, one should also not overlook the effects of pressure and humidity
on our brain, ears, sinuses, etc.
Paul
