Does sound travel faster in high pressure or low pressure?

Sound travels best when it's in something which is tight - consider the "telephone" using two cups and a piece of string. When you warm anything up, it becomes more "loose" as the molecules become excited. (Ice is hard until you melt it. Tar is pretty stiff unless you heat it up.) So it follows that sound would travel *better* in a cold environment. There would be better transmission of the wave from one molecule to the next, and the molecules would typically be closer together, so it would theoretically travel a little faster. However, if the medium is air, this also depends hugely on the air pressure. As the pressure decreases, the molecules become more sparse (less dense), and it's more difficult for an affected molecule to pass on the waveform. So sound at the top of Everest may not travel as quickly as sound in the middle of the desert, despite the obvious temperature difference.