Your big speaker housing is nicely drawn, but you should remake the rounded rectangle hole in the back. It is not cleanly drawn, and you can’t repair what you have - start over with the hole.
First delete all the faces, stray lines and edges round the hole. You’ll have to zoom in very close to see some of them. And separately, you have a lot of stray edges round the rectangular socket on the right of the recess. Delete them too.
Then draw a rectangle (looks to be about 11" wide and 2 3/4in high), round the corners with the Arc tool, and offset the resulting face inwards by the thickness of the wall you want it to have - say 1/16in - to start creating a recess for the sockets.
Then pushpull the thin 1/16in wide edge further than the deepest part of the recess.
Make the piece into a component, move it into to place, then explode it into the same context as the speaker housing. Use Intersect Faces/With context to cut the hole in the speaker housing, and trim off the excess on the outside.
I’ve tried that, and it didn’t work well - some small edges were created and holes appeared in the main speaker housing, I think because my recess component was slightly too small to cover the existing hole in the housing.
I’d suggest starting from scratch again with a new housing with no rear holes, then follow this procedure from a clean start.
For what it’s worth, here’s the recess component I created.
Recess.skp (40.3 KB)
And what it looks like, put in place before exploding and using Intersect Faces
If the housing had been complete i think this would have worked.