A tip o' the hat to reader Frank C., who sent me the link to a site named 'More Cowbell'. It allows you to upload an MP3 file of music, set controls to add the sounds of cowbells and Christopher Walken (speaking phrases from his movies) to the soundtrack, then play it back. It's quite comical, and strangely addictive!
Here's a sample: Art Garfunkel's theme 'Bright Eyes' from the movie 'Watership Down', with '50% cowbell' and '51% Walken'.
You can also access songs uploaded by others. There's quite a collection already on the site. I can tell you that Led Zeppelin's 'Stairway To Heaven' is a whole new experience with added cowbell!