But be careful with NM-B cable. I didn’t find an exact reference in the NEC, but I bet that Article 200 and friends would not approve of the reuse of the bare conductor for anything other than ground and might also disapprove of using the white conductor for anything other than neutral. (If you’re not installing the cable in the wall, maybe it’s not subject to the NEC.)
Get some nice TC cable instead.
Where the NEC applies, you can use the white conductor in a multiconductor cable for anything as long as it is "permanently reidentified", that is color coded where it is exposed. cf NEC 200.7.
I am not suggesting you dual-use your high voltage romex and also send audio signals on it ...
I am suggesting you run additional runs of romex dedicated solely to audio.
This would be NEC compliant.
Red and black are fine for this purpose. I’m not so sure about white and bare.
The reason for the 4 AWG rule is probably an economic one. Having spools of larger conductor cables in a variety of colors is not inexpensive. Allowing the reidentification seems reasonable, though I'd probably argue the cutoff could be thinner than 4. The multi-conductor cable exception is a practical [and economic] one. You might not be able to get multi-conductor cable assemblies with the exact color coding you need.