Yes indeed, it really looks like there is a problem in accessing the configuration register when it is in a subslot (at least other than a -0 slot). Of course it can still be a specific issue of this type of slot expander
The strange thing is that in version 1.2 it did work (be it probably only for 2MB) and in 1.3 not, even though it is also still detected as a MoonSound.
Although i now have a reasonable workaround by just putting the DalSoRi in subslot -0, i still want to try some more things and try to find at least what's really going on here 
Let’s narrow it down some more… Add ld a,"x", call System_PrintChar after the first jr c,NotFound in DalSoRiR2_MatchSlot to determine exactly in which step it fails. And a call System_PrintHexA after the pop af in OPL4_Detect.
Also additionally I was thinking maybe there is some timing issue, although it doesn’t seem likely. You could try to intersperse ld b,0, djnz $ in OPL4_Detect before and after each call.
I tried again on my turboR and CX5MII with my MK expander btw, I still can’t reproduce it on either 
.
Actually i already tried exactly the same with 'X' 
The 'x' is never printed so disabling the port does not work, the pop af results in 30.
Double checked again in slot -0 and there the x is shown just before detection and pop af results in 20
I indeed don't think it's a timing issue since i see it also on my MSX2+. Next to that OPL4 detection is not a a problem, it even detects when it should not 
I did try it with the additional delay loops, even disabled the R800 but still the same effect
Found the problem, and as it turns out it indeed was the slot expander itself 
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It has 2 jumpers (JP4 and JP5) which can be used to duplicate subslot 1 on 0 and subslot 3 on 2. Apparently the jumper for subslot 1 and 0 was not working: i swapped it with the other one and now subslot 0 and 1 work, 2 as well but subslot 3 not. Swapping it back result in subslot 0, 2 and 3 working but subslot 1 not.
So for me the reported problem with the DalSoRi 2 no longer reproduces as well...
That certainly explains it. Quite a peculiar feature those jumpers offer, it's clearly a Brazilian made expander I think MegaRAM was popular there, whereas here few people have one. I wonder what the jumpers do exactly. Anyway. It sounds lile even with a broken jumper the slot can still be used for an I/O only device.
Now that leaves Pencioner, I wonder why he got the same symptoms too.
@ Grauw - no, I'm not having it. I had posted that my tests were ok, i can't reproduce:
I tried to reproduce the DalSoRi R2 issue but at my setup it is working fine in slotexpander, tried both Modulon and 8bit4ever one - all is good. But i don't have Turbo R - i tested on Sanyo Wavy 70FD. So might be that the issue is not related to either DalSoRi or VGMplay - it might be some hw related stuff
EDIT: and my guess about HW issue was correct 
Oh ah it was Avkooi .
@Avkooi does all other hardware (both I/O and memory based) work well in those slots where the DSR2 has problems? E.g. Musical Memory Mapper if you have it, or SCC / FM-PAC / any megaROM.
