We found out that sound is a kind of wave that is caused by the air vibrating, or moving back and forth.  If all sounds are caused by vibration, how come all sounds don't sound exactly the same?  How come some sounds are high pitched, like Mariah Carey hitting those high notes, and some sounds are low, like deep voices and bass guitars?

The faster the vibration,  or back and forth movement, the higher the frequency.  The higher the frequency,  then the higher the pitch will  sound.  The slower the vibration, or back and forth movement of particles in the air, the lower the frequency.  The lower the frequency, then the lower the pitch will sound.

This is what a sound wave looks like:
http://www.silcom.com/~aludwig/images/snake.gif

Take a look at these waves.  This is why sounds do not all sound the same:
http://sln.fi.edu/fellows/fellow2/apr99/soundvib.html

Here are some more experiments you might want to do:
http://www.sci.mus.mn.us/sound/nocss/activity/ssl3.htm  http://www.sci.mus.mn.us/sound/nocss/activity/ssl9.htm  http://www.galaxy.net/~k12/sound/groovygu.shtml