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

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.