[jimmaswell]

Frequency is just periodic variation in volume so it doesn't really belong on that list as if it's a separate thing and not an emergent property

[sharpneli]

They are complementary. You can fully describe sound with either function of volume in time or complex amplitude (So normal amplitude + phase) in frequency. So you could just as easily say the volume is emergent property of the frequency.

Regardless it's an 1 dimensional function.