What speaker wire should I use to extend the length?
I would just install 14 gauge wire, it will be more than adequate at 30 feet.  

Look for a reputable brand that's made for audio and rated CL2 for in-wall installation.  Any decent speaker wire will be made with fine strands and sufficient purity for your surround system.  

You can spend a lot more on speaker wire, but you won't hear any difference.