https://buildd.debian.org/status/package.php?p=ocaml-fdkaac&suite=sid claims ocaml-fdkaac has been in state BD-Uninstallable for a while on all architectures except amd64 because of missing libfdk-aac-dev, but https://buildd.debian.org/status/package.php?p=fdk-aac&suite=sid tells that a suitable version should be available on all architectures except arm64, powerpc and ppc64el, and all other build-deps should be there as well.