This isn't everything you've asked for, but here's how to open and play an audio file (mp3, wav, ogg) within your game:
In a new scene add an AudioStreamPlayer and FileDialog node as children of the root node. Since we only want to open files set the FileDialog's mode to "Open File" in the inspector. FileDialog nodes can't be visible by default, so you'll also want a Button that will make it visible whenever you press it.
Once you have a script attached to the root node you can connect the Button's pressed() signal and the FileDialog's file_selected() signal to the script. The following code should get everything to work:
$FileDialog.visible = true
var song : AudioStream = load(path)
As an FYI the volume might be really loud by default, so turn it down using AudioStreamPlayer's Volume Db property in the inspector.