only one file? That could be it also. When the page loads it maybe looking for the other 9 wav files to play and not playing anything because there's no wav file. But it will play once it finally randomly hits that one wav file..
Since you're only using one wav file, instead of using:
$sound = iif($bbuserinfo['pmunread'], '<embed src="http://www.washmeclean.com/downloads/sounds/message'.$srand_snd.'.wav" autostart="true" loop="0" hidden="true"></embed>');
try using:
$sound = iif($bbuserinfo['pmunread'], '<embed src="http://www.washmeclean.com/downloads/sounds/message1.wav" autostart="true" loop="0" hidden="true"></embed>');
or change message1.wav to the name of the wav file you're using(testing) and see what happens.