I heard (and tried) adding a small container (i.e. film canister) to the end of the air hose will quiet it down. I tried this a while ago so I may not be remembering all of the detail correctly but I think I drilled one hole in the bottom of the canister and another in the lid. The air tube from the skimmer was placed in the bottom hole. Result: little to no reduction in sound... maybe a slight change in pitch. So, then I took the lid off and that made it louder then ever. I did try this with several containers some larger some smaller. But to no avail.
I would like to try this again and have a few ideas but before I hack up a bunch of containers wanted to see what others have done to successfully quiet their skimmer.
Here are a few of my ideas:
instead of a single intake hole have a bunch of smaller ones
loosely pack the container with filter floss, cotton balls or something similar
Go with a much larger container (quart size or larger)