use function to stop your sound.
ex:
var play;
SOUND* sample_sound="sample.wav";
function sound()
{
play=snd_play(sample_sound,100,0);
}

function sound_stop()
{
snd_stop(play);
}

function main()
{
screen_color.blue=250;
on_b=sound();
on_s=sound_stop();
wait(1);
}