[SOLVED] The problem was the battery leakage. Because of that, the patches and sequencer memory was lost. The Poly 800 was making strange sounds or almost no sound at all.
The same problem was described in this youtube video:
SOLUTION: I've changed all the batteries (6 x 1.5V). And reprogrammed the memory by loading into the Poly 800 the default or factory patches from the wave file which can be found here:
Hope this would be helpful,