Hi! I really wonder if this is still worth fixing... This package is dead upstream (upstream vanished, every attempt to contact him again failed), and I am essentially keeping it on life-support here. It also has a relatively low popcon index, and I announced that it is threatened to be removed about a year ago, unless maybe someone take over upstream maintenance or forks it. Additionally, mp3gain isn't present in Debian anymore, only vorbisgain, which makes this package even less useful.
So tl;dr, I think removing the package from Debian is sadly the better option than fixing yet another FTBFS. If there are still users, maybe a different maintainer who actually still uses this package and can fix functional issues can take it over. I will leave this issue open for a while, maybe wait for the package to be removed from testing and wait for feedback - if there is sufficint interest, I might revive the package even. Otherwise, I intend to drop it from Debian in a month. Cheers, Matthias