I am overwriting data, not adding. I even checked to make sure my edited SDAT is the exact number of bytes (13,289,472) as the original. NDSTS still says "File size incorrect!" ...
EDIT: Turns out the SDAT must be in the same directory as NDSTS. Importing worked...now to test the ROM...
EDIT:
SUCCESS! I've got Planet Puzzle League (U) working without background music!
I can't thank you enough, FAST6191. I couldn't have done it without you. For anyone interested, I set the first two highlighted column values in Post #9 above so that they are just out of range of the SDAT file. Here is a screenshot, showing the values I changed:
Note that setting these values will disable all music in-game and on the menu. The splash music at the title screen remains. If you edit the values below the red highlights you will eventually disable sound effects, which I did not want. Also keep in mind that the file size must be 13,289,472 bytes when you import the SDAT back into the ROM. Feel free to play around with it yourself, or PM me with questions.