Well, yes and no.
There was a fairly significant write-up of the MML in the "Develo"
Assembler and BASIC kits for MSX/PC-9801 computers in Japan.
In the Assembler kit, there was a comprehensive write-up of how it works
(in Japanese), and an MML ("Music Manipulation Language" ?)
assembler/player. You can see some of this information reverse-engineered
into English on my PC-E info page:
http://www.interlog.com/~daves/pce_info/info.html
In the BASIC kit, there was an actual "PLAY" statement which would play MML
directly.
I have also seen references on some other Japanese sites to MML, and some
MML utilities for UNIX machines, but I have lost most of the references.
Anyway, this implies that it was not limited to the PC-Engine, but rather
it was used in a greater context - probably starting from the MSX days.
As for the big software houses, I think they used similar tools for
arranging the sounds - similar to those described in the Develo books. It
appears that the PSG music player was one of the first pieces of code
written for the PC-Engine (and it MAY HAVE been ported from the NES), and
it was probably licensed to the original developers to be included in their
games. Later, it was included in the CDROM card.
If you read the write-up on my info page, you will note the distinction
between the MML "notation" and the byte-code stream that is interpreted by
the PSG player.
>are there any utilities out there for programming the TG (or a TG emulator)
>besides basic assemblers? specifically, I'm interested in doing PSG music
>programming.
>
>how was this done in the big software houses? somehow I doubt all the music
>was done through raw assembly coding...