Good question!

I don't know if it may work or not, but you could try for plugging in your n900 to your computer in storage mode, and run a recovery soft on it.

I'm gonna make a little test right now to see if it works.

EDIT:

I just tested deleting a file (a video file) from the file manager, plugged in in usb storage mode, and run Recuva on it, and i could recover the file, also with some other deleted music files.