Neon -> 2,090mph

Krypton -> 2505mph

Xenon -> 2,438mph

http://www.wolframalpha.com/input/?i=speed+of+sound+in+neon

http://www.wolframalpha.com/input/?i=speed+of+sound+in+krypt...

http://www.wolframalpha.com/input/?i=speed+of+sound+in+xenon

But even Neon (which is more abundant in the atmosphere than Helium) would cost ridiculous amounts of money to use.