Is it okay to use an expired cake mix? Will it affect the baking?

I would throw it out. It has fat in the mix which can go rancid, and the baking powder loses its effectiveness over time. The cake mix is usually the cheapest part of making a cake, so just buy a fresh one!

It depends on how long ago expired.
If it's for a client, definitely don't use it. But if it's just for your family, I've used expired boxes many many times, and it worked just fine. I think one I used was over 6 months expired.

I agree w/KateLS.
It definately depends on how old it is.
Possibly it will not rise properly.
I'd use the usual look & smell test