Problem with sound and buttons

I have 4 sound files on the level. All of them are labelled. And I have buttons on my navigation bar with transitions to that labels.

button 1:
on(release){
stop();
stopallsounds();
gotoandplay(“a”);
}
button 2:
on(release){
stop();
stopallsounds();
gotoandplay(“b”);
}
etc.

everything works great until I click the 4th(last button). If I click after that button 2 or 3, sound starts to play from the middle.

Can somebody help me?