I have one similar to yours, the
octopus bh-2000. my silencer has 2 holes too, but I noticed that if you insert the tube one end is louder and the other quieter, so the other end is still open. The blue sponge I do not use I only use the black sponge (although I'll probably be getting rid of it soon b/c I don't see a point except for micro bubbles). I didn't have the siphon version so I'm not sure what the blue one is for. Usually for test and trial the best way is to just keep plugging it in (with the body full of water of course) and trial and error. good luck!