Basic timing testing method:

	; a = register
	; d = value
	WriteRegister:
		out (MSXMusic.ADDRESS),a
		REPT 13
		nop
		ENDM
		ld a,d
		out (MSXMusic.DATA),a
		ret

Reducing the REPT by 1 causes music playback glitches.

To test e.g. LD A,(IX), I did this:

	; a = register
	; d = value
	WriteRegister:
		out (MSXMusic.ADDRESS),a
		ld a,(ix)
		REPT 13-7
		nop
		ENDM
		ld a,d
		out (MSXMusic.DATA),a
		ret

In the REPT, this value would not glitch, and if I changed the 7 to 8, it would.

Similarly, to test CALL, I did this:

	; a = register
	; d = value
	WriteRegister:
		out (MSXMusic.ADDRESS),a
		call BLAAT
		ret
	BLAAT:
		REPT 13-8
		nop
		ENDM
		ld a,d
		out (MSXMusic.DATA),a
		ret

Changing the 8 to 9 would cause it to glitch.