Just another thought. I looked it up. At 25 degrees centigrade the speed of sound in :

Water = 1493 m/s
Mercury=1450 m/s
Pyrex Glass=5640 m/s
Rubber = 1600 m/s
Diamond = 12,000 m/s !!!!

Mercury is over 13 times denser than water. Why is the speed of sound lower in mercury? Pyrex glass, while heavier than water, is not 4 times heavier. Why is the speed of sound in Pyrex Glass that much faster?

The speed of sound in any substance is governed by how well in "contact" the molecules are. In a gas, the molecules are "in contact" at a rate governed by thier temperature.