1. Closed-cell foam (the same as for a backpacking mattress) underneath the sump.

2. Closed-cell foam underneath the return pump.

3. Connect the return pump to the pvc pipe with a length of flexible hose.

My Mag 12 return pump has been silent this way for years.

I used that stuff from J& L mentioned above - its very porous and a medium hard plastic sorta mesh - works well as a bulkhead to deflect/disrupt water flow from say a skimmer outport or beneath falling water - its quite thick.

Ive had good lluck with those exercise mats, the interlocking kind.
They look like the same things swimming pool toys are made from.
Anyways my ocean runner 6500 was always noisy till i used that foam under it.
Also i found moving it further away from the sump sides made a big difference.
the sides amplified the sound.

I had an issue with this and just covered the bottom of the pump with silicone. Now I sleep right beside it and its fine :)

i use neoprene from an old pair of my waders