Ernie's suggestion about twisting the Romex sounds decent, until a little analysis of what you're really dealing with...

Here in North America, power is 120V 60Hz. You're certainly not going to change any impedance matching characteristics, and therefore the coupling characteristics, by putting a couple twists for every 5 feet. Even if you were to twist it 100 times every 5 feet, it's not going to make much if any difference at 60Hz. 60Hz is practically DC, and twisting will have very negligible effects on coupling due to electric or magnetic fields at these frequencies. It might affect the ability of the audio to get onto the AC, but I don't think we're worried about that!

The biggest factors are coupling, and proximity. Normally you handle this by using spacing, but this isn't a great option. So that leads to changing the coupling.

If you are worried about coupling, you could run shielded speaker cable. Another possibility might be balanced audio cables, with the amplification built into the speakers. (I don't if know if speakers with balanced inputs to an onboard amp is common, but would certainly work from an electrical perspective.) The other possibility is to shield the AC. This could be done by using BX (Now referred to as Type AC in the code).

I would think any of those would work, and the BX might be nice anyway, as it will prevent a nail from puncturing the AC when putting up the moulding.