FYI to anyone interested. Mednafen, the emulator author, fully uncovered how to activate the "Ys IV: Dawn of Ys" Debug menu some time ago and I demonstrate how to reveal the sound test!
Details here:
https://www.pcengine-fx.com/forums/i...?topic=18808.0
And she later made a Cutting Room Floor entry all about it:
https://tcrf.net/Ys_IV:_The_Dawn_of_Ys
I was able to use the Easter Egg to activate the PSG sound test by talking to a certain character in the Developer's Room as well.
I detailed the attempt here:
https://www.pcengine-fx.com/forums/i...0129#msg400129
You have to make Music Supervisor Takimoto (The Invisible Man) unhide himself basically by equipping the Black Potion, standing where he's hiding and then he'll give you access to the sound test. Anyway, this was a bit of a tangent but there you go!
EDIT: My video [finally] shows you how to fully unhide Takimoto to access the sound test, full demonstration.
https://youtu.be/45VV2_D2kgw