Amplitude of a signal. [-1, 1] vs variants and MFCC implications
What do these values actually mean? Are they arbitrary?


Yes, pretty much: they will just be the scale of numbers your system works with. In floating point systems, we often see that samples get normalized to [-1,+1], whereas in fixed-point system, it's often things like [-2⁻¹⁵,+2¹⁵-1], depending on the bit width of the samples to begin with.

So, this is purely conventional. Most algorithms don't care about the absolute scale of things.

So, if your algorithm does depend on amplitudes, well, normalize your signal before you process it.