Not directly that I know of, save for some scraps from the official Sony SDK currently on archive.org (if you're willing to use that, anyway; it refers to the whole system as "ezMIDI" in said docs). With that said, the developers of vgmtrans appear to know enough about SQ/BD/HD that openin...

Hello there! I'm looking to inquire about how the SQ, BD, and HD file formats work the on PS1. As well as, indeed, what they're even doing there, because up until very recently I always thought that those were formats specifically for the Sony SDK's sequenced music engine on the PS2 (you might know...

This looks interesting from the perspective of someone who would love to see hombrew sequenced music running on either the PS1 or PS2 (preferably the latter, which is why I'm glad to see the source code for this program for that console here). I'd personally prefer either MIDI or its PlayStation SDK...