BBC Learning Zone Broadband Clip Library
The Learning Zone provides rich audio-visual material for use in primary and secondary schools and colleges. These short videos have been selected to match the curriculum; they can be used in many ways, from the stimulation and engagement of students to the delivery of very specific learning points. Try searching by subject and topic or by keyword. Link
I tried looking for clips on sound and found two useful videos showing sound travelling through water and air. I would use this site again.