If we look at a tuning fork vibrating back and forth, we see that it has areas of high pressure (compressions) and low pressure (rarefactions) continuously forming and moving away from it.  This is what we know as sound.  How do these form?

1.  You strike the fork and the arms bend in. They have elastic potential energy at this point:

 

2.  The arms spring out, compressing the air immediately next to them.  Only the air on one side is shown for clarity here:

 

3. Having sprung out a bit farther than is stable for the metal in the tuning fork, the arms spring back in, creating an area of low pressure right next to the tuning fork.  Meanwhile, the molecules in the region of high pressure have banged into the ones next to them, and the compression has moved on:

 

4.  The arms spring out again, creating a new compression:

 

5.  The arms spring in again, with a new rarefaction forming right next to them:

 
 

6.  The arms spring out again, creating a new compression; the other compressions have moved on as before: