@Yerzmyey : the chip is called "HuC6280A", and it is both the CPU and the sound processor, a all-in-one chip in the same way as the gameboy.
(and it also looks very nice with the bee logo)
It's a PSG square waves generator, similar to the AY (minus the envelope which can be found on AY), but with 6 channels and some sample capabilities. I'm using it in a very simple way, like for most of my tunes, but there are very "advanced" musics for this chip (given the fact it's a simple 8-bit chip).
@th4 D34D: thanks! I try to add some tempo variations in a few of my tunes.