How to open media browser from tinyMCE window?

I'm trying to create a shortcode to embed audio files, with custom data as copyright owners and year, etc. It's a basic tinyMCE window with textboxes to insert the information, and the URL of audio files uploaded to the media library.

What I need is a way to create a button on this window; when clicked (onclick), open the media library, so I can find audio files and when insert it, populate the textbox "audio_url".

My code by now (not working properly, see below):

{ // this is only one option from a lisbox where my other shortcodes are...
    text: 'Audio Shortcode',
    onclick: function() {
        var win = editor.windowManager.open( {
            title: 'Insert Audio',
            body: [
            {type: 'textbox',
            name: 'audio_url',
            id: 'audio-url',
            label: 'File...',
            value: '',
            },
            {type: 'button',
            name: 'find',
            text: 'Find...',
            onclick: function() {
                var audiofield = win.find('#audio-url');
                var myurl = tb_show("Audio search", "media-upload.php?type=audio&height=700&width=600&TB_iframe=true");
                editor.windowManager.open({
                    url: myurl,
                    width: 700,
                    height: 600,
                    resizable: 'yes',
                    close_previous: 'no',
                    oninsert: function ( url ) {
                        audiofield.value = url;
                    }
                }
        )},
    },              
],
onsubmit: function( e ) {
    editor.insertContent( '[audiomb src="' + e.data.audio_url + '"]');
    }
});
} },

So it will show a textbox where the file URL must be, and a button to open the media library, where I can find a file and insert it - what will populate the field.

It's not working because the tb_show iframe loads behind the tinymce window, and an empty window in front. How can I make it appear on top (focus), them browse the media gallery to get the file URL? Do tinymce has a better way to work on it (instead of tb_show)? Thank you all.