The C64 has three audio channels, the NES has four. Native C64 games were designed with that in mind, but this port has to double up the NES' triangle and noise channels on one C64 voice, so sometimes a sound effect gets overwritten. The article discusses it.

I loved my C64 as a kid and played if for about billion hours. I loved the music - some of my all-time favorite "songs" are from C64 games (Ghost 'n' Goblins, Commando themes are sooo good!).

When I revisited the C64 as an adult the first thing that immediately jumped out at me was the dropped/stuck notes that happen in a theme song when you get a powerup or trigger some sound effect.

I can't believe I never even noticed it as a kid, and my brain just filled in the missing sounds!

The C64 only had three audio voices (compared to four on the NES), so it was difficult to play decent music and multiple sound effects at the same time. A lot of games let you switch between music or SFX, but not both.

Even so, its musical capabilities were years ahead of anything on the market. I usually played in music mode, if it was available. :)

There's a netradio station that exclusively streams remixes of C64 game music. It's been broadcasting for at least 20 years. It's my favorite thing to listen to while coding. https://www.slayradio.org/