Scholastic
Scholastic: Dirtmeister's Science Lab: Good Vibrations
An experiment that explores the relationship between vibrations and pitch. This site includes web links, a challenge question, and notes to the teacher.
Other
The University of Salford: Sounds Amazing
This site has several lessons all about sound. It introduces sound, then covers major principles of waves and the wave equation. Each lesson has interactive activities to keep you interested, and there are even some test questions.