Shallow angles on the hoses will reduce the noise considerably in the hoses. Also so will slowing the flow. I have 2 return pumps in my sump, and if I shut one off the flow is slowed by half and it gets a whole lot quieter.

I went through a lot of PVC pipe trying different configurations for my in sump plumbing (good thing its fairly cheap). The angled one in the picture was the quietest. Believe it or not, some of the other things I tried actually made it louder than the pipe going straight in. One thing I tried was just drilling an air hole in the straigt pipe just above the high water mark. It sounded like someone was constantly peeing standing over a toilet even though it got rid of the bubbling. (Not good, at all).

I have the PVC screwed to the lid of the sump so it cant fall out, but all of the plumbing in the sump is not glued together to make it easy to disassemble to maintain. I figure worst case is if it comes appart it will still be in the sump so no mess, it'll just make a bit of noise until fixed. Haven't had a problem with that yet though.

Another thing I found after quieting the sump, was that one of my return lines was making a little noise because a little water was leaking from the anti-sypon hole. I just moved it closer to the side of the overflow so that it was spraying onto the plastic just above the water level instead of directly into the water in the overflow column which eliminated another noise.

It's still not completely silent but it is much quieter than my other tanks with HOB filters when the water level gets a little low.