External Interface not working?

In my flash I have this (AS2) :

function playSongJS(str:String, sta:Number, sto:Number):Void {
 // do things
}
flash.external.ExternalInterface.addCallback("playSongJS", this, playSongJS);

Then I call it like this:

function getFlashMovie(movieName) {
    var movie = document.getElementById("ap1_mod");
    return movie;
}
function playSongWithTime(my_file, start, end) {
    getFlashMovie("ap1_mod").playSongJS("test.mp3", 0, 50);
}

I include it using SWFObject:

<div id="ap1_mod"></div>
<script type="text/javascript">
    var flashvars = {
        mp3Path: "stop",
        artistName : "",
        trackName : ""
    };
    var params = {
        codebase: 'http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=9,0,0,0',
        src: '/public/flash/ap1_mod.swf',
        quality: 'high',
        pluginspage: 'http://www.macromedia.com/go/getflashplayer',
        scale: 'showall',
        devicefont: 'false',
        bgcolor: '#999999',
        name: 'ap1_mod',
        menu: 'true',
        allowFullScreen: 'false',
        allowScriptAccess:'always', //sameDomain
        movie: '/public/flash/ap1_mod.swf',
    wmode: "transparent",
    allowfullscreen: "true"
    };

    swfobject.embedSWF("/public/flash/ap1_mod.swf", "ap1_mod", "400", "50", "9.0.0", false, flashvars, params);
</script>

This is not working and I get a JS error saying that playSongJS is not defined.

I don't know what could be wrong and I've already wasted too much time on this. Any help would be greatly appreciated

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