The high frequency sound generated in helium is because the speed of sound is about 3 times as fast in helium as it is in air.

http://en.wikipedia.org/wiki/Speed_of_sound

where

- c
_{ideal} is the speed of sound in an ideal gas.

- R (approximately 8.3145 J·mol-1·K-1) is the molar gas constant.
- k is the Boltzmann constant

γ (gamma) is the adiabatic index (sometimes assumed 7/5 = 1.400 for diatomic molecules from kinetic theory, assuming from quantum theory a temperature range at which thermal energy is fully partitioned into rotation (rotations are fully excited), but none into vibrational modes. Gamma is actually experimentally measured over a range from 1.3991 to 1.403 at 0 degrees Celsius, for air. Gamma is assumed from kinetic theory to be exactly 5/3 = 1.6667 for monoatomic molecules such as noble gases).

- T is the absolute temperature in kelvins.
- M is the molar mass in kilograms per mole. The mean molar mass for dry air is about .0289645 kg/mole.
- m is the mass of a single molecule in kilograms.

http://en.wikipedia.org/wiki/Adiabatic_indexThus we observe that for a monatomic gas, with three degrees of freedom:

while for a diatomic gas, with five degrees of freedom (at room temperature):

Thus to reduce the speed of sound further, you probably require a gas with a large number of atoms, and a high atomic weight.

http://members.bellatlantic.net/~vze2n9fe/sound/sound.htm[tqble]

Gas | Density (kg/m3) | Speed (feet/sec) | Standard Deviation (feet/sec) |

Air | 1.2929 | 1039.7 | 22.4 |

Argon | 1.7837 | 957.8 | 20.3 |

Carbon Dioxide | 1.977 | 812.4 | 23.0 |

Propane | 2.009 | 771.5 | 21.3 |

So there would be some slight drop in frequency when breathing propane.

According to

http://www.mysolv.com/AGA10CalcOut.asp, butane would have a speed of around 208m/s, compared to the 235m/s quoted for propane above, and compared to around 331m/s for air (the table above seems slightly lower).

I would guess that most molecules that are heavy enough to have a speed of sound

^{1}/

_{3}^{rd} the speed of air when a gas, would probably be too heavy to be gaseous at STP.