Matlab plot help

I'm worried this is a very very silly question but that means it'll probably be easy to answer...

I've imported a very simple .wav file into matlab (it's just four notes playing one after the other) and I have to write a function to count the time inbetween the peaks in amplitude.

When I plot the wav file I get a graph and my very basic silly question is, is it showing amplitude vs time? It looks like it is...

 

That depends on how it is plotted. wav files are stored as waveform over time. You can see the amplitude in the waveform, though.