The song called Wakka's Theme was only found in the sphere theater. It never played in the actual game otherwise.

It's hard to say if Seymour is really the big bad or not, but he is a nice embodiment of both the corruption of Yevon and the "spiral of destruction" theme. Like Sin, he just keeps coming back and can't stay dead. In the end, he wanted to become Sin. He's the only thing you fight actually actively trying to cause destruction and foil the plans of your party and whatnot. I'm not sure who would be the big bad other than him unless you just want to say there is no real big bad. Outside of when Jecht could have control, Sin didn't really seem like it had a mind of its own. It was just mindless destruction. Yu Yevon was much the same by this point I think too, but there isn't much to go off there. Yunalesca was a part of the corrupt Yevon culture that was perpetuating the final summoning tradition, but that's it.

Anyways, Wakka is a cool guy. I've always liked his story arc.