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Study chest voice or low register vocal ranges

As a bonus, eventually you would knock them out on karaoke night.

The timbre of your voice depends on the vibration of your vocal cords.  The slower they vibrate the deeper the voice.  When you get a cold, the cords become inflamed and  thicken, slowing them down and deepening the sound.  Inhaled steroids used for asthma treatment often lower the voice, but you can also thicken your vocal cords, by smoking, shouting or catching a cold.

In the same way you can get a high pitched voice by breathing in a low density gas like Helium, you should get a lower pitched voice if you breath a higher density gas.  Since air is about 78% Nitrogen (molecular mass 28kg/kmol) and 21% Oxygen (mm 32 kg/kmol) you need something substantially more dense than either of these, and commercially available.

I would suggest Xenon (mm 131 kg/kmol) to get a deep sexy voice.  But - and this is important - you need to stand on your head when doing this otherwise you'll never be able to expel it from your lungs and will drown!

It may be that standing on your head while breathing Xe would reduce your sex appeal rather than increase it, but you never know until you try...