It's going to be in the wall, so you start being subject to Building Regulations (as this a new build). Which, frankly, is a bit of a pain, but there you are. But there you are.
I have Chord Campana Speaker Cable in my walls and its excellent
Specification Multi-strand, oxygen free copper conductors
Strong, flexible polyethylene insulation
Parallel conductor geometry
For custom installation use
Discrete and easy to install
Ideal for multi-room use
Easy to terminate
its about £2.30 ish a meter, so wont break the bank.