var speed = 0;
var accel = 1;
var boundPos = 200;
function bound() {
    this._y += this.speed;
    if (this._y>=this.boundPos) {
        this.speed *= -1;
    } else {
        this.speed += this.accel;
    }
}
function xSetAnimation(_mc) {
    _mc.speed = speed;
    _mc.accel = accel;
    _mc.boundPos = boundPos;
    _mc.onEnterFrame = bound;
}
xSetAnimation(ball1_mc);
xSetAnimation(ball2_mc);

var speed = 0;
var accel = 1;
var boundPos = 200;
function bound() {
    this._y += this.speed;
    if (this._y>=this.boundPos) {
        this.speed *= -1;
    } else {
        this.speed += this.accel;
    }
}
function xSetAnimation(_mc) {
    _mc.speed = speed;
    _mc.accel = accel;
    _mc.boundPos = boundPos;
    _mc.onEnterFrame = bound;
}
ここで、更に関数を作成
--------------------------------
以下のスクリプトで動作させる。
btn_mc.onRelease = function() {
    xSetAnimation(ball1_mc);
    xSetAnimation(ball2_mc);
}
もう一つの書き方は、更に関数を作成して
function btnStart() {
    xSetAnimation(ball1_mc);
    xSetAnimation(ball2_mc);
}
以下のスクリプトで動作させる。
btn_mc.onRelease = btnStart;