0003A50D: 03 18
0003A50E: 03 18
Adjusts two consecutive entries in a data table. Those extend the last two states in the Ether Medallion animation to make them last 8 times longer.
00067E6F: 1F 05
Modifies the immediate operand of a LDA instruction. This lowers the intensity of the flashing colors that tint the title screen as the sword is dropping into place. Instead of cycling between tints of full intensity red, green, and blue, it's now cycling between dull red, green and blue.
00075805: 0E 02
00075818: C0 40
00075819: 01 00
0007582C: 38 08
Modifies immediate operands to ADC instructions in a routine that adjusts colors to gradually tint more towards white. IIRC this is used when the seal on Ganon's Tower is being broken by the Crystal Maidens. The tweaks put in place cause the flashing process to take about 7 times longer.
000DE03C: 72 70
000DE03E: 9C 95
000DE040: FF 57
These modify the lower bytes of entries in a large table full of palettes. The first two reduce the red component, and the last change reduces the red and green components of a color. Which palettes? It would take more effort I'd rather not do at the moment to figure that out. Best I can say at the moment is that they are palettes meant for BG tiles in indoor parts of the game. Specifically, my notes mark them as "DungeonMain BG palettes (6 in each group)"
I already basically investigated these several months ago b/c someone in a Zelda IRC channel asked me about them. I figured they would have put that information up somewhere, guess not?
I am curious to know how this music pack mechanism thing works though. The emulator program would have to monitor something to know when to load the right track and sound effects. And I would also assume that they'd have to emulate the SPC-700 to at least some degree to not break anything as there are infinite loops where the S-CPU awaits responses back from it.
Edit: It does appear that all of the other changes were to data that gets fed to the S-APU, and it is a massive quantity of changes. Maybe that's how they accomplished this trickery? Presumably the "APU" in the Wii U is just a fake and the program being uploaded is for a simpler virtual device that plays the appropriate sounds / music when fed the right numbers? I don't have that rom so it would be hard to deduce whether it's real SPC-700 code and data, but I am leaning towards probably not based on what you've said.