My understanding is exactly the opposite -- that separate strands (of the
correct total cross sectional area) are better than a single wire.
That's
because of "skin effect" (tendency of high frequencies to be carried at
the
periphery of the wire, although that is probably negligible at audio
frequencies) and also because of better heat dissipation.

Can someone elucidate?

Skin effect is really really important if you have any real program
content
you care about in the upper radio end of the spectrum

Like I said, "negligible at audio frequencies"...

I mentioned it because it's not a reason to *avoid* using multiple strands.