Without the change the build fails on clang and upstream gcc as:
$ nix build --impure --expr 'with import ./.{}; wavegain.override { stdenv = clang12Stdenv; }' -L
...
ld: /build/main-51a0e2.o:(.bss+0x310): multiple definition of `file_formats';
/build/audio-d7ad83.o:(.bss+0x8): first defined here
ld: /build/wavegain-ea60c8.o:(.bss+0x140): multiple definition of `file_formats';
/build/audio-d7ad83.o:(.bss+0x8): first defined here
