LN4890 is NatLinear's clone of National Semiconductor LM4890. It's pin compatible, and from your pictures you can see that the speaker is directly connected across its outputs.
http://www.natlinear.com/en/AA.html
The 5800C looks like the small FM ICs found in many PMPs, and although its pinout is different from the '5767, there's this FM module with a very similar chip:
http://wiki.s1mp3.org/Image
xt_5807_chip.jpg
...and this AR1010 also seems to have the same pinout (coincidentally, this FM board looks exactly like the ones in our MPx).
http://www.sparkfun.com/commerce/pro...oducts_id=8770
It's quite safe to conclude that it's the FM receiver.