I've only heard what they sound like from the episode where they are caught, but I know Cacnea sounds kinda like Elmo. A really sweet voice. He says his name clearly.

Seviper is kinda like a hiss and some cracking noises. Kinda like Arbok, he doesn't repeat his name clearly.

I still haven't heard what either of them sound like in battle. I have a feeling they'll keep Seviper's Japanese voice. Cacnea's has to me changed for obvious reasons (although I wish they wouldn't, I really like his voice! ) As long as they keep him sounding innocent without sounding annoying (I'm looking at you Phampy) I'll be happy.