getElementsByAttribute

document.getElementsByAttribute = Element.prototype.getElementsByAttribute = function(attr) {
    var nodeList = this.getElementsByTagName('*');
    var nodeArray = [];
    for (var i = 0, elem; elem = nodeList[i]; i++) {
        if ( elem.getAttribute(attr) ) nodeArray.push(elem);
    }
    return nodeArray;
};

Snipped from
http://stackoverflow.com/questions/9496427/how-to-get-elements-by-attribute-selector-w-native-javascript-w-o-queryselector

usage would be
document.getElementsByAttribute('data-foo');

But the following is preferred for browsers that support it
document.querySelectorAll('a[href$=".mp3"]');