Is speaker type wire 20 ga. ok for led undercab lights buried in walls? from 12v trans.

what variety of speaker wire? does the wire have an in-wall (FT4) rating? Run length? load?

20ga sounds extremely small

I’d say no. Unless the low voltage wire is listed as CL2, you’re really not supposed to conceal it in the walls, etc. That’s how it is enforced around here anyway.
The last LED kitchen lighting job I did had both in cabinet and under cabinet lighting. What I did was run the switch leg down to some J-boxes I mounted the circuit panel location in the basement and then fished a 14/2 NM up the wall to each cabinet light location from there. I mounted the drivers for them down there as well.
IMO, the benefit of using regular NM for low voltage under cabinet lighting is that you don’t have to worry about the situation with concealing it in walls and also if for some reason the customer ever wants to go back to line voltage lighting, you’re all set.

Off top of my head, with out see Mfg instructions, I would have to say no. Unless it is a complete listed and labeled for the application.