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+

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