Not sure whether it’s too late for you guys as I just saw this thread.
I owned a 500 and a 600. With the 500, same sound faded out and sometimes in-and-out, especially using the XPS. Eventually I fixed the problem by spraying WD40 over the entire circuit board and all switches (you need to spray a lot like soaking it with WD40). It’s been over a year and still working good no matter with battery or the XPS.
With the 600, no sound with battery or XPS. I did the same trick by spraying WD40 on the mode selector (soaking with WD40) and the same miracle happened.
If the faulty 500 still with you guys, it’s worth to give it a try as i worked for me.