There are other parameters apart from temperature.

In laymans terms, s.o.s is greatest at sea level - or where air density is greatest.

As you gain altitude the air becomes less dense untill you actuall thin out into space, where density approaches zero ( over 400,000 ft ).

Gravity keeps dense air down - obviously.

Temperature causes air to rise because it expands the 'mass' making it 'lighter'.

I have no data on how moisture content effects s.o.s