This is happening to My NXT 2.0 brick as well, did you get a solution for this ?
It was working well until some time back , the brick stopped making any sound a couple of days before , i do not remember if i did any thing different.
I have installed the LeJos firmware. I have tried increasing the volume on the setting to maximum , but no luck.
I was using the sound class to PlayTone , it was working for some time before it stopped suddenly , is it possible to break the internal speaker by sending wrong values to the PlayTone method ?