+1
Drywall is cheap and easy to fix. This is not youre mistake and if the client really wants that James soundbar then someone needs to pay to get this fixed, otherwise you start cobbling together some weird science project that when it has issues everyone will point fingers at you.
Btw you should speak to the builder about hiring you to do his prewires as it is obviously more difficult than he realizes and is above his pay grade.

I have a client that is getting ready to open up his brand new basement ceiling because he changed his mind about the tv location.
Not my problem, but I am going to do what it takes, to do it right.