Quote by BiMale73(b) refers to (a), when (a) is not defined, it is impossible to understand what (b) is about.
Sure, but (b) can exist, and be a valid definition of sound, without (a) being defined as sound. That's why I said that both are valid definitions on their own. They don't rely on the other to be a definition of sound as well, to be a valid definition of sound themselves.
Auditory sensation evoked by something we do not need to define?
If we write out (b) it says:
Auditory sensation evoked by an oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or by the superposition of such propagated oscillation.
enjoy the view 
