I usually buy the infinity edge for voli, and build up AD. Is that a better idea than AP?

his w scales off maximum health.

his passive is percent health.

Why build inf edge, when warmogs would be better for damage and survivability?

tank more, please.

Yeah, Warmogs is pretty standard Volibear. Infinity edge is nice, but there are better choices. Try building up health as much as possible early game. If you still need more AD, you can always build an atma's impaler later on. I personally like frozen Mallet because of both the health and the help with ganks

^I agree.

Besides your boots, pretty much all of your items should have something to do with survivability (mainly health)

Tri Force is good
Iconic Spark is awesomeness, it's like a mini ult and it gives you atk spd and health.
Frozen Mallet is good with the sticking power (but you don't need trinity and FM that's overkill.)
Atma's is nice as well.

Warmog's is pretty much a must-have on Volibear
Spriit Visage also works exceptionally well with his passive and how you build him.

Volibear doesn't need a ton of dmg items to be effective. Health should be the priority.

Go all out tanky. Throw in a Phantom Dancer for speed attack