I asked two dealerships (western Canada) and was told by both that adding an aftermarket sound system WOULD void the warranty. However, they also stated not to take their word for it 100% and recommended calling Kia Canada. I will be calling them soon...
You might wanna check the last couple of thread. there's even a stikied thread. Aftermarket parts don't void warranty. However if the after market part caused damage to a part, then YOU have to pay to fix that part.