Although Bitt is right that the sled might be suspect, first check to see whether it is the simple known software bug.

There is a way to verify whether or not it was the software bug.

On mine, when it does this, I can clearly tell that the DSP is muted in an "opposite-toggled" state because of this behavior (quoted from my internal bug report):

"When the player is on and it's not making any sound, I can sit there with the volume up and see the time counter ticking up and the blinking dot moving across the bar on the screen. Then, if I press the Pause button to pause the player, I get a brief "burst" of the song out of the outputs, then silence as it pauses. (Actually, don't remember if the burst was on the pause or on the unpause, but you get the idea). "

So the next time you see it playing without making any sound, try to pause/unpause it with the volume up and see if it makes the brief burst of the song. If that happens, it's the exact same bug, and it's a software bug.