newbie with simple sound problem

[bansche]

Hello,
i think there is a loop in my head and i couldn't fix a simple problem.
I want to start a sound in a movieclip under a simple condition like here:
//Width = 600
onClipEvent(load){
a = 4;
this._x = -582;
this._y = 117;
}
onClipEvent(enterFrame){
this._x += a;
if (this._x > 171)[
a = 0]
}
so if this._x exceeds 171 or != the sound should start.
I tried a lot, but at all times the sound starts at the beginning of the MC.
Thank You for helping.
Best wishes from Germany
Bansche