Some worlds' custom sound formats aren't supported

[SpaceManiac]

1. 32 custom sounds are "RIFF (little-endian) data, WAVE audio, MPEG Layer 3" which is not directly supported by SDL2_mixer. It can load WAVs and MP3s, but not WAVs that have MP3s in them: Support wav-files with MPEG encoding with mpg123 libsdl-org/SDL_mixer#377.

   17 affected worlds: expedition1.dlw, expedition2.dlw, expedition3.dlw, Halloween4EM.dlw, halloween5hr.dlw, Halloween5MZ.dlw, Halloween6HR.dlw, hh5bp.dlw, hh5jr.dlw, monthly11.dlw, monthly2.dlw, monthly4.dlw, monthly5.dlw, monthly7.dlw, Spamumu.dlw, ss2JB.dlw, WinterWackiness4HR.dlw

2. 1 custom sound is "Sun/NeXT audio data: 8-bit ISDN mu-law" which SDL2_mixer has no decoder for. However the format is simple and we can write our own decoder.

   1 affected world: vacationacres.dlw