What I do is this;
1) My drain line is vinyl hose, which goes from the drain then connects to some PVC, a 90 degree bend brings the flow to the left of my sump to where my skimmer secion is.
2) At that point, the drain is now pretty much horizontal, with a slight downward slope.
3) that piece hits a PVC tee, with the other openings of the tee pointer up and down.
4) On the UP part of the tee, I put yet another tee, so it looks like the vent at the top of your house with a rain guard
5) The submerged side of the first tee is submerged.
This helps a lot and is sort of like your "waterfall/splashguard" idea.
Basically what happens is that the drainwater comes out sideways and hits the T where water and air goes up and down. The splash that is going upwards hits another T (with the other openings going sideways). This blocks many splashes and bubbles (even more if you connect this T to PVC that gives it more height). The water, of course, goes downwards and underwater, so it's not as loud or splashy as an open connection.
It works great for me, and I've tried different variants, but the idea is the same...divert the water sideways, then smash it into a T so that water can go DOWN and air/splash can go UP. The UP side has a splash/sound guard and the DOWN side is submerged and discharges water. You can try perforating the submerged part too.
Id does make the top part sort of like an "open air skimmer" like Gorm said, but it doesn't cause any problems for me.
I'd take a pic, but the sides of my sump have stuff growing all over them and that section is cramped with the skimmer output sitting on top of the drain part!
Oh, and my hinges are mostly rusty too, even though I attempted to paint them!
Also, to make life easier, I painted the entire inside of the cabinet with white primer so that light would reflect around in there and make things easier to see.
V