I've never heard of Soundnails before, but after a quick look on their homepage I now know what it is!

It seems to me like it is designed to be played via a button by default. The only way to change that would be to do some particularly difficult Flash involving importing their SWF generated file, finding the code that starts the sound playing and triggering it. Oh and of course, hiding their interface in some way. Of course, you would need Flash to do that and the entire point of Soundnails is for people who don't have Flash - so doing all the stuff I said above would be completly pointless.

If you do have Flash then you might as well just import your SWF file and make the sound manually. That way it can start immediately if you like. If you're confused for details then please let me know.