sgu to wav conversion is actually mere export of Band-in-a-Box midi songs (.sgu) to standard PCM audio format (.wav). The original Band-in-a-Box does not unfortunately support export to WAV, just import of .wav files.
Various MIDI scores can be exported as standard audio with MuseScore notation software for example and this might actually be the best option here, considering the limited support for .sgu files.
Updated: October 10, 2023