COPYRIGHT
If you plan to put your sound clip online, you will need to follow basic copyright guidelines.
You may use a section of any copyrighted piece as long as it is less than 30 seconds long, is not the main chorus of the song, and has been properly credited (this is a summary of several interpretations of Copyright Law, and is not an actual rule, but this is a guideline that is easy to remember and follow. Use your discretion).
Common information to include when crediting a music source is Artist Name, Composer, Title of Composition, and Record Label and Number if possible.
Even if a musical work has passed into Public Domain, Public Domain only covers the music, not the performance of the music. When music is recorded, there are additional rights given to the performer, the director or producer, and the writer.
Shake off all doubt, and give credit where it is due!
Even Fair Use has very restrictive guidelines on music use.
For more information, try:
Guidelines for Educational Uses of Music
Copyright Tutorial
Fair Use Guidelines for Educational Multimedia