Which are the differences between these 4 FM-PACs (forget the PAC on the top-left)?

How many different FM-PACs were produced in The Netherlands?

The FM Stereo PAKs have stereo output (obviously ) and come with the same ROM as internal MSX-MUSIC's, which means only FM BASIC is in the ROM.
The original FM-PAC is the only one with the CALL FMPAC program and has ofcourse the PAC SRAM.

Do they have the same sound quality?

The dutch produced FM-PAC clones are pretty good AFAIK. Not that I speak from experience, but I can't recall hearing any complaints. The Korean FM-Ship is known to be very noisy.
However, to my knowledge, none of the them have the volume switch (low, mid, high) that the original FM-PAC has.

The FM-PAK (the stereo variant) has a built-in program as well. Even though the FM-Basic can be used on MSX1 too, the built-in program requires at least MSX2 and can be called with CALL FMPAK.

I suppose it's a different internal program than the original one?
And, more importantly, is it dumped and does openMSX support it?

I've a FMPAK cartridge and also a rom image of this cartridge.

I think that the rom image runs correctly in openMSX, but I must check it ...

openMSX doesn't seem to support the FMPAK cartridge : I've tried with Catapult and I get the message "Syntax error" when I use call fmpak ... so, it means that this rom is not included in the openMSX rom database ...

In my preferred emulator (NLMSX), no problem at all : call fmpak gives access to the integrated software