Only this one worked for me:
<script type="text/javascript">
var frames = document.getElementsByTagName("iframe");
for (var i = 0; i < frames.length; i++) {
src = frames[i].src;
if (src.indexOf('embed') != -1) {
if (src.indexOf('?') != -1) {
frames[i].src += "&wmode=transparent";
} else {
frames[i].src += "?wmode=transparent";
}
}
}
</script>
I load it in the footer.php Wordpress file. Code found in comment here (thanks Gerson)