Switch from OGG/Vorbis to OGG/OPUS
Ilmendur currently uses OGG/Vorbis for music files. It has been argued that the newer OPUS codec is a better choice than Vorbis, so it makes sense to have Ilmendur use OPUS instead of Vorbis.
However, both codecs usually come within the OGG container. The audio library used by Ilmendur, SDL_mixer, is capable of decoding an OGG/Vorbis container using libogg and libvorbis directly, but it is unable to do the same for OGG/OPUS files. It instead relies on the opusfile library to do that, and as of the writing of this ticket, the opusfile library is not stable. Until either SDL_mixer supports loading OGG/OPUS without opusfile or opusfile becomes stable, it is hence not acceptable to switch to OGG/OPUS instead of OGG/Vorbis.
In addition, a short build attempt revealed that as of version 1.3.1, libopus does not compile when using cmake:
Make Error at CMakeLists.txt:19 (include): include could not find requested file: opus_buildtype.cmake
It can be compiled with autotools, though. Maybe the libopus maintainers should remove cmake build support to prevent confusion.
This ticket serves as a reminder to check the above problems every now and then, or alternatively to decide ultimately against OPUS.