CKEDITOR.dialog.add('insertsoundDialog', function(editor) {
    return {
        title: "Insert audio",
        minWidth: 300,
        minHeight: 150,
        contents: [{
            id: 'tab',
            label: 'Settings',
            elements: [{
                type: 'text',
                id: 'url',
                label: 'Please enter an MP3 URL to insert:',
                validate: CKEDITOR.dialog.validate.notEmpty("URL field cannot be empty.")
            }]
        }],
        onOk: function() {
            var string = '<audio controls><source src="' + this.getValueOf('tab', 'url') + '">';
            string += 'Your browser does not support the audio element.</source></audio>';
            editor.insertHtml(string);
        }
    };
});