This addon substitutes the Lil' XT sound files
(excluding the explosion sounds but including the Ulduar XT sounds)
with blank sound files. This is so everytime Lil' XT triggers a sound file, you will hear nothing.
A blue post claimed to have hot-fixed it but people still complain that it is still making sounds.
Unzip the folder into your \World of Warcraft\Data\ folder (Not the addons folder)
There's probably more XT sounds than Lil' XT makes, but it was too much of a headache just listening to which specific sound files he uses.
Credit goes to the TrainWreck mod (By sensory)