Hi. It seems like you have one of the earliest Obsidian sound boards. I would do that same for this one as I do for my version v3. I block the inside end of the pommel where the sound holes are. Then make holes in the material to partially let the sound and heat out. What works best for me is the round floor protectors that are put on the bottom of chair legs. Are inexpensive and can be purchased at dollar stores. The sponge ones would be easier to put holes in than the felt version.
There is a muted sound font available that is on the newer v4. You should be able to download this for free. However you will have to go on to the Obsidian computer program to change the sound out every time unless you are wanting just a muted saber for awhile.