SPC700 I/O ports. Write values to play music/SFX; see details lists for valid values in each port.
The original game reserves $1DFB for music, while $1DF9 and $1DFC are used for sound effects. $1DFA meanwhile handles a couple of additional high-priority sounds and special effects (e.g. toggling Yoshi drums).
Note that the valid values lists are based on a clean rom. If using a tool for custom music, songs and sound effects may be remapped for their original layout. For example, with AddmusicK you can find them in "Addmusic_sound effects.txt" ($1DF9/$1DFC) and "Addmusic_list.txt" ($1DFB).
