“The pedestrian warning law isn't in effect yet, nor could I find anything in the bill ( http://thomas.loc.gov/cgi-bin/query/D?c
... 11UTU44d::) that says it's illegal to disable/change the sound. It just says that the manufacturer cannot make it so the sound can be disabled. It says nothing about us changing it.”
Disabling a safety device provided by the manufacturer may not be illegal (criminal) but it sure can be used against you in a lawsuit. It may show a callous disregard for the safety and lives of others.
The expression of concern over a blind friend who was unable to hear the warning sound can justify modifying the device to be more audible but going the other way is IMO unwise.