I tried using Zahlman's editor to switch the Pallet Town theme with Pallet Town Fame Remix, but when I loaded the rom the song was the same. I received this output from Zahlman's editor.
Also the Fire Emblem theme was the same when I tried the example:
# This sample script assumes you have an Advance Wars ROM named 'AW.gba', and a Fire Emblem 7 ROM named 'FE7.gba'.
# It rips Sonja's theme music from Advance Wars and replaces the opening theme from Fire Emblem 7 with it. The result is saved to 'test.gba'.
# After that is done, it will dump all the instrument samples found in the FE7 ROM to a folder called 'samples', in AIFF format.