Sound is a mechanical
wave - it propogates (moves along) by using the vibrations of particles in matter (as opposed to electromagnetic waves that don't use particles in their propogation!).
Sound energy is transferred
through matter by passing on vibrations of particles in that matter via a longitudinal
wave. The vibrations made by those particles passing on the sound wave are parallel to the
direction in which the energy is travelling.
Wavelength (symbol λ -
lamda a Greek letter 'l') is the shortest distance between two corresponding parts of the waveform. For a soundwave that means the shortest distance between two particles
of the medium the sound is travelling in that are oscillating in phase. (Shortest distance between two particles
that are moving in step with each other). It is measured in metres (m)
Period (T) is the time
taken for one complete oscillation of a particle in the wave.
It is measured in seconds (s).
Amplitude (A) is the
maximum displacement from the mean position.
Frequency (f)
is the number of oscillations per second. It cannot be read directly
off a graph. You need to find (T) from the displacement/time graph and
then find its reciprocal (f =1/T)
See
the waves section for details on the correct naming of wave terms.
You may find this video interesting....
I would prefer you to think of the density of air molecules being altered as sound travels through the air rather than you thinking individual molecules get stretched and squeezed... this video has some good bits and some not so good... see if you can spot the all the dodgy bits!
See
the waves section for details on wave terms - make sure you know them all.
You may also find these interesting:
Corn starch is a 'shear thickening non-Newtonian fluid' meaning that it becomes more viscous when it is disturbed.
When it's hit repeatedly by something like a speaker cone it forms weird tendrils.
The video below was shot at 30 fps and the speaker cone was vibrating at 30 Hz which is why there is no blur.
This is the original video with the actual sound of the speaker.
Here is an interactive Java activity:
The following site covers sound really well - take a look at it.
I would prefer you to think of the density of air molecules being altered as sound travels through the air rather than you thinking individual molecules get stretched and squeezed... this video has some good bits and some not so good... see if you can spot the all the dodgy bits! 
The following 
