Why did you measure inside the cabinet? Don’t you want to dampen the perceived noise outside of the cabinet?

Physically speaking, 3 cm will absorb all frequencies above 12000 Hz, and dampen everything beneath at least a bit, ignoring the absorption capatbilities of the material. I’m no expert in the field of material sciences, I just know by word and experience (insert “trust me bro” here) that its absortion capabilities are really good.

It will lessen the perceived sound by a measure, but do not believe, that all sound will be gone. But a 3 cm-thick paddening all around will to a good job.

Think twice about glueing. Glue is nasty to remove, maybe just push cut-to-size pieces inside of the ducts to make them fit? You could make ones, which you make snug around the cables leaving the cabinet?