Typically, with the Flash Rich Media Annotation (RMA) you’d set the RMA to play when the page that it’s on is either opened or visible. For movies that play on the page, you may want them to start when clicked or automatically when the page is first viewed. But with Sound RMAs, at least most of the ones that I’ve seen, the player opens in a floating window so that users can browse through the PDF file while the music is playing.
In the "Actions" tab of a link, bookmark or button, you can specify a "Multimedia Operation" and then a variety of actions like "Play", "Pause", "Rewind" etc.
this.getAnnotsRichMedia(this.pageNum).activated = true;
Why does this work? Well – the Sound RMA player is set to autoplay when activated. By activating the Sound RMA, you are, by default telling it to play. After the Sound RMA is activated, other Multimedia Operations will work as expected including "Play" if the Sound RMA is paused.