God Slayer sounds terrible on my famiclone so I wonder is there a special chip in the game that the console can't use properly?
It's an MMC3 game, I don't think MMC3 has anything special that Famiclones may have problems with but I don't know for sure. There's no sound chip in MMC3.
I guess your Famiclone just isn't very good soundwise. Some CPU clones has swapped the 25% and the 50% duty cycle settings swapped for the Square channels, which Gode Slayer is using a lot.
Try the game in an emulator like Mesen that allows swapping the two duty cycle settings, to simulate these clones, and see if it sounds the same as on your clone. In Mesen you go to Options -> Audio -> Advanced and check "Swap square channels duty cycles (not recommended)".
https://www.mesen.ca/