I don't think you can play with duration of a sound.. That involves synthesizing.. And flash can't do that. You can play with a duration of a pause. though.
It can only play straight sound.
try making a really big array like mySoundArray = new Array(1000);
And create a tempo is a setInterval... the tempo will go through each index of the array every say 1/10 of a second...
if an index == 0 then don't play anything...
if an index == a sound then play that sound.
here's an example