Just installed Mincraft 1.4.2 and now getting major sound lag from running water blocks. I have a single player creative flatlands stage with 42 water blocks each placed over an iron block.

FPS:
My normal FPS is 60-90. Flying around these water blocks causes a reduction of FPS to 6-40, and walking causes fps of 6-150, creating stuttering graphics.

Debug "sound" and "tick" spikes during lag:
My normal debug sound is 1%, and tick is 15%. While flying and walking around water blocks, debug graph shows the "sound" category spiking 25-30%, and tick spiking to 40-50%.

Disabling sound eliminates the lag:
In the Options menu, turning SFX off eliminates the lag completely. Leaving SFX on and reducing all graphics options to minimal does not eliminate lag, only slightly reduces it. Only turning SFX off completely eliminates lag.

Lakes and rivers without elevation change (no "water" sound) don't cause this problem:
I've testing walking around lakes and rivers with no elevation changes that would cause the water sound with SFX on. Lag is not present around still bodies of water.