This is true. Only the ending BGM uses the FDS synth. It produces very vibrant audio, I love this sound chip.
But this is false. Or maybe it isn't, it depends what mod you're talking about. I'm only aware of the mod that connects the missing audio channels, which would allow games with sound chips to be audible on the NES. However the FDS' FM synth is not present in an NES system. So either you're wrong or there is now a mod I don't yet know about that allows you to solder a compatible synthesizer sound chip into the NES. (Similarly I know there's a mod to solder a Yamaha FM synth chip into the US SMS but the system was supposed to have support for it to begin with. The NES was not. I'll let you tell me what is the case for NES and FM synth.)
Thanks. It's not so much about skill though. It's not that hard to learn the stages and have the skill to navigate traps. The challenge was getting the entire run perfectly. As I hinted I have so many low-death runs it's not even funny. In Japanese we'd say 運ゲー which literally means Luck Game. There are a few elements across the game which I literally depend on luck to accomplish. The last hammer brother in 8-1 comes to mind. The two underwater stages and two stages with flying fish both require just being sharp and smart about where you're going. Hell, this run was nearly ruined but I got a little lucky, right around that spot where I accidentally ate that mushroom. (Grr.)
7-4 is by far the most difficult Koopa to beat. It's insanely dangerous the way it's set up. In my run I cut it so closely he could've taken Mario's hat off. XD