i suspect your current applet's implementation of the sound
sound1 =flying up
sound2 = explode

does not coincide with real fireworks
for example i check this video
<iframe width="425" height="349" src="http://www.youtube.com/embed/xF9I-9bzyhE" frameborder="0" allowfullscreen></iframe>
the sound starts from xi after immediate after t2=t-delta

can check? i found a bug i think after i set delta = 0; the flying and the sound1 does not coincident