how do I tell embedded sound to stop playing on certain page of pdf?

Ok - I haven't actually tested this code but I think I got it right. If you can send me the file in question, I can check it.

Anyway. Since  the user can activate a rich media annotation in a variety of ways and you can't reliably get a handle to which ones one active, you need to turn every one of them in the document off before starting the one on the page you want.

Add this code to the "Page Open" script for each page with a sound. The code will go through the document page by page looking for sound annotations and turning them off when not on the active page. Then it will turn on the sound on the current page.

// iterate through all the pages in the document and deactivate any playing sound files
for (var i=0;i < this.numPages;i++) {
  // chech that we're not on the current page
  if (i != this.pageNum) {
  // check if the page has a richMediaAnnotation
  if (typeof this.getAnnotsRichMedia(i) !== "undefined") {
  // assuming there is only one sound file
  if (this.getAnnotsRichMedia(i)[0].activated == true) {
  this.getAnnotsRichMedia(i)[0].activated = false;
  }
  }
  }
}
// Activate the sound on the page the user is on. 
this.getAnnotsRichMedia(this.pageNum)[0].activated == true;