Construction around a recessed slab

I need to run CAT6 Ethernet cable from my basement into my audio cabinet where my router is. The basement is under the main part of the house, the cabinet is in the living room which is on a recessed slab adjacent to the basement.

There's 6" between the floor surfaces, I assume ~1" of flooring in the main house and the bottom of the cabinet is 3/4" plywood. That should give me ~4" from the floor of the cabinet to drill through to pass my CAT6 cable. I'm not sure what I'm going to have to likely drill through, however.

The back of the cabinet is probably 3/4" plywood, I can see a 2 x something from the basement and I assume there's drywall behind the cabinet. Is that it, or is there likely more that I'll have to drill through?

I've attached a sketch of a section through the house in that area to maybe better show what I'm trying to do.

Thanks.

Approximate House Section
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THe 2x is the rim joist. If the part that has only a slab on grade is an addition they may be some type of sheathing between the house and the addition. That is likely not a big deal and shouldn't be difficult to drill through.

 

Thanks for the help. It's not an addition, part of the original construction in the mid 80s.