-these both need to be considered. One law ( Boyle's ) states that P x Volume/T = A constant.
P=pressure, T=absolute temperature

Thus all aspects effect each other.

If you heat something the molecules 'move' faster, and so s.o.s is faster too. But the pressure increases and in the real atmosphere the air will 'spread out' and become less dense ( it is not kept in a sealed 'box' as per lab experiment.

As density drops, so does s.o.s.

Temperature has a major effect in real terms as we often fly in all weathers. Pressure? sea level is about 1 Bar ( 14.5psi ), and down to zero when you leave the earth's atmosphere!