SDL_mixer cannot play MPEG Audio files that don't start with a frame header

[Fulgen301]

ClonkPartyRemake.c4s uses MPEG Audio files that start with 417 null bytes, which corresponds to an entire audio frame of null bytes. The reason for this is unknown, but they still constitute a valid MPEG Audio file since implementations are expected to search for the first occurence of a frame header, which SDL_mixer currently doesn't do.