I think this is a common misconception. Cooler (denser) air molecules move more slowly than warmer air molecules, and that effect negates the 'packedness' that typically results in denser=faster sound. For ideal gases, denser=slower sound.

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe3.html

No, you're thinking of absolute humidity.

Lower (higher) temperature for the same amount of water vapor results in higher (lower) relative humidity.