JavaScript not working across browsers

[MDLarson]

I'm trying to setup an image toggle that turns on and off a background WAV file. Here's a working example:
http://www.hazmatt.net/test2.html

Safari 3.2.3 (Mac) works as expected.
Firefox 3.0.10 (Mac) image swap works but sound stays on.
Firefox 3.0.10 (Windows) works as expected.
Internet Explorer 6 (Windows) image swap fails and sound stays on.

Is this kind of spotty support typical with JavaScript, or am I doing something wrong?

I'm also open to alternatives, as long as the interface is identical to what I already have.