Play Sound After X Seconds?

I have built an admin area for employees of a business. Once logged-in, if they sit idle for 5 minutes their session will be killed ( the time may vary depending on the employee ). I do all this with php but don't really know much about javascript.

What I would like to do is start playing a small sound repeatedly when the employees session is about 30 seconds from expiring. ( this will give the employee time to click on a link to keep their session alive )

So when the page loads, I would like to pass a length of time in seconds to a javascript. So if the employee can sit idle for 5 minutes, that would be 300 seconds. Minus the 30 seconds that I would like the alarm to be playing for which gives us 270 seconds. ( I'll pass this number to the script with PHP, the script does not have to figure this out... just to be clear. )

So I'm looking for something like this:
- On page load, tell javascript to count down a specific number of seconds ( 270 )
- When the countdown reaches 0, start playing sound file repeatedly.

Sounds easy enough but I haven't been able to find anything like this yet. The only ones I've seen actually display the countdown which I don't want. I want this to count down invisibly. Oh and it should hopefully work in both IE and Firefox.