diamondback cobra 24 rear brakes squeal horribly what can I do to at least tone it down

Often this is the result of poor "toe-in." As the wheel turns while braking, the brake pads' mounting posts flex slightly and the pads buck against the rim and bounce back, causing the squeal you're experiencing.

The fix is to adjust the brake pads so the frontmost end of the pad is slightly closer to the rim. This makes it so that the pad is pulled square with the rim as it makes contact. The pads' post should affix to the brake arm with a bolt and a dished washer. Loosen that bolt, rotate the pad slightly, and retighten. Repeat for all the pads. This bike has V-brakes, and this can be tricky to get right.

I have a Surly Long Haul Trucker which had the same problem on the front brakes.

My current theory is that the cause is a cheap rim that has its brake tracks on left and right sides separately machined. Separate machining means their "waviness" doesn't match each other. If they were machined at the same time, they would be "wavy" but in an identical manner on the left and right sides so the "waves" would match each other.

My fix currently has been to use a different brake pad compound. I really wanted to use Kool Stop salmon compound but it squealed like hell when dry and when wet. Then I found the Shimano S70C compound squeals only slightly when wet and not at all when dry.

Someday when I have enough time I'll build a new front wheel for the bike using DT Swiss TK 540 rim that is far more expensive than the "Xtreme Sari T-19R" rims (not for sale anymore, bought from German Rose Bikes originally) that cost less than half what the DT Swiss rims cost.

I'll see whether the Kool Stop Salmon compound works with the TK 540 rims later.

And the obligatory note: yes, I tried several different toe-in settings. No amount of toe-in did anything to silence the brakes.

I also tried Kool Stop Salmon and Dual Compound pads, both as separate shoes, and as pad material for the Shimano S70C shoe holder. All of these options squealed like hell.