Well, most Romex is solid core which would make it quite inflexible (which means a pain in the *** for installs). The ground, which won't even be used, is bare and exposed, so it's just asking for trouble. Plus, because of the shape, I think it wouldn't be very easy to work with. I'd suggest getting some good stranded speaker wire. Belden makes some good 10ga and 12ga stuff. You can also find some nice white-jacket speaker cable at Impact Acoustics.
Now, Romex won't sound inferior (provided there isn't something electrically wrong with it), but it'll add more hassle to your job.