In a vacuum s.o.s = zero.

So in your air tank experiment, as you say, if you take half the air out, and keep the temperature the same, the s.o.s is unchanged??

If so, and you then removed another half of whats left ( leaving 1/4 ), and another ( leaving 1/8 ) etc.... at what point will the s.o.s drop?

Sound needs molecules / atoms to propergate which is why both temperature ( velocity / excitation of atoms ) AND the number ( density ) of particles are important.

This is also why the s.o.s is higher with increasing density of whatever the medium.

ie, s.o.s is faster in steel than in > water than > air > vacuum...

Please correct me if I'm wrong...