I'm not sure about making it damage reduction (though if I did, it'd be overcome by Sonic), simply because then it would be uniform (and TTO's resistances aren't).
The separation of feats might be a good idea, though. I might even add a separate area for Bonus Feats, just to make it more clear where everything comes from.
I'm not sure how DR would be different to resistance in this instance, given DR/- works the exact same way as resistance does. You also couldn't have DR pierced by Sonic, since DR can't block energy damage, that's what Sonic Resistance would be for. It's just putting the information in the correct places.