In the ROM, the code for the sound driver starts around $ED60B-ish, and I'm fairly certain the actual data being sent to the SPC700 is directly above that. It looks like the spc data in the ROM is compressed (could be wrong).
I can't help much more than that currently, but if you run a trace while the game boots up, it will load the data for the first two songs into APU RAM.
Just check for writes to register $2141 and check above that in the code to see where the data is coming from. Typically, you should see it transfer the music data from a ROM (ex. ~$4ed5e4) address into a RAM (ex. ~$7f75aa) address-- especially if it's decompressing data-- before storing it in APU RAM. You'll want to convert all addresses referenced in the disassembled code from HIROM addressing to PC addressing (the ROM address I just mentioned would convert to ($ED5E4 PC). Just from quickly looking at the code, this should be where your search should start.
Hopefully that will get you started. Others may wish to chime in that have more experience with the APU. Feel free to correct me if any of my assumptions are wrong as well.
Unfortunately, this is about as much as I know about how the APU functions as I've not had to deal with it much before. Good luck, my friend.