First, edit each button and add a shape in the Hit Frame to give your button a correct "hot spot", ( hit frame stays invisible in the movie), your button is only responding to the graphic shape at the mo.

i would take the sounds out of the button timeline.
open the Library and give each sound a Linkage Identifier.
and use the sound(object) on the button actionscript,

on(rollOver){
s =new Sound(this);
s.attachSound("IDname1");
s.start();
}

hth