Unable to play high frequency mp3 in Storyline 3

Hi all.  We have a presentation that is demonstrating the human hearing range (20Hz - 20kHz).  We have 2 sound samples (mp3) that play a sound at 15kHz and 20kHz; the idea being that younger people can hear the higher frequency and older people can only hear the lower frequency.

The issue I have is that after putting these sounds on the timeline they don't play (preview or published).  I can put other sounds in and they work fine.  I can play both of these mp3 files outside of Storyline and they work.  And, before anyone asks, I got a young person to confirm this, cos I can't hear either of them!

These sounds were part of a swf that happily played in a Presenter presentation previously.

I am wondering if there is a limitation in Storyline to do with the higher frequency and how sounds are processed, saved and presented that means that these can't be heard?

If anyone has any information on this I'd be very happy to hear it.  

Thanks

Paul