Best way to turn the sound off on an obsidian lite board is to buy a closed/sealed pommel from the online store under the parts section. This would work a treat to knock out 99% of the sound.
Cheers.
Actually US has a video of their duel sabers with sound strongly cautioning the use of a a sealed pommel as it will destroy the speaker, the voice coil will vibrate and crush itself under it's own pressure. Uncertain if this would happen to just a single saber.