Am I the only one who thinks that Norwalk doctored the audio tapes of Kleinfeld?

It’s possible, but I think Kleinfeld sold out Carlito. All the coke, the booze, the high life; and remember, he burned the mob guy, Tony T, for a mil. I see Kleinfeld having no problem providing info on Carlito to save his ass.

But you’re right, Norwalk was dirty

Carlito (past self) was not a good guy, he was dealing copious amounts of heroin and most likely involved in multiple murders to facilitate this. I think the judge’s words were something along the lines of “reputed assassin.” Norwalk may very well have been operating honorably and with good intentions…, we don’t know that he was dirty, he may have just had giant fucking balls and was tired of seeing his city be consumed by murderous heroin dealers.
That being said, the Carlito we are introduced to is not the Carlito as described, he’s clearly a changed man.
Given Kleinfeld’s nature I see no reason to take Norwalk’s word that Kleinfeld sold him out as anything but true.

Possible but I think Kleinfeld's reaction when Carlito confronts him about it tells him everything he needs to know

This film is a work of fiction. You are coming up with theories that was never filmed on screen. You are acting silly

While Norwalk certainly seems like the type, considering the shady & illegal tactics he used to get Carlito convicted previously but Kleinfeld more or less confesses with his "save your own ass" speech.

"the reason why Carlito was in prison to begin with was because Norwalk had pulled some sketchy and less than legal strategies when it came to tapes/evidence".

That brings up an interesting point.

Why didn't they sue!?