+1. OFC is OFC, and its current handling limits are based on its cross sectional area and length of the conductor. The main differences I see are based on the insulation, strand count (and thus strand gauge) and whether or not the wire is pre-tinned.

Now, if 12 ga speaker wire is smaller than 12 ga RC wire (which it shouldn't be, as the gauge should be standardized across all manufacturers), then I'd agree with idea that 12 gauge speaker wire will not handle as much current.

Another thing to keep in mind is that speaker wire manufacturers might spec a lower current due to the lenth that is being assumed. If you look up some standard wire gauge specs, you'll see that for a 12 gauge wire, max current is usually spec'ed at 40-45A. We run a LOT more than that, but the wire is so short that it doesn't make much of a difference.

IIRC, conventional wisdom is that you want no more than 2% voltage drop over the length of the conductor when calculating max current capacity. For efficient systems, you can try for 0.5% to 1% voltage drop.

Even with 12 gauge wire at 120A for a wire that's only 3" long, with a 4S battery, the voltage drop is only 0.32%. It's even better with 6S at only 0.21%. 10 gauge is better at 0.20% and 0.13%, respectively.