Can I use 11 speed chainrings with 9 speed chain? To be more specific, my bike come with Shimano Sora 9 speed group set. I'm using a KMC X9 9 speed chain.

I studied quite a lot of websites. They claim that chainrings are compatible with any speed of cassettes.

Are there compatibility problem with existing components if I put on Shimano 105 11 speed chainrings?
Basically, so long as the front derailleur is wide enough to carry a 9 speed chain that you're using (so a 9 speed front derailleur would work) without problems, you'll be fine. 

So, go for it, provided they'll fit (i.e. the bolt pattern + BCD matches). 

As for mismatching the chainrings, chances are these are the cheapest ones on the market now. My road bike, for example, runs a Shimano 105SC groupset (7 speed) and I'm going to have to replace the chain rings with things that are "10/11 speed" eventually.