water is basically incompressible. unless it's turning into ice or vapor, it stays at VERY close to 1 g/cm3.
on the other hand, water viscosity changes quite a bit when heated. gal's discussion of laminar vs. turbulent flow is relevant--even if you don't have turbulent flow conditions when pouring hot water, you'll get smaller droplets forming when pouring hot water, making more of a drippy drip sound, than the plopping sound of large cold water drops.
here's a boring flash link:
http://www.grow.arizona.edu/water/wa...perature.shtml
viscosity is the answer. trust the scientists here.