Some games (like Super M) uses special audio chipset

A GR8NET or Carnivore2 would get the job done quite well using a single slot and having sound expansions for most games, with the rare exception of games that use OPL4 like Life on Earth. Surely FM emulation can get some improvements, I think that the recent PCM like hacking on FM only works on real chips and do not work on the FPGA simulations, hopefully they will update it at some time to fix it

If I'm not mistaken, GR8NET has MSX MUSIC as well, Eugeny was working on that, so a GR8NET and a OPL4 would cover you for almost anything that is a game, and OPL4 is not much used, so unless you really want that bad, I wouldn't recommend buying one unless you have other plans (like playing SID or MOD files on MSX, as OPL4 kind of allows that).

About MSX slots, usually the two cartridge slots are primary slots. Each primary slot can be expanded to 4 sub slots, some devices like GR8NET, SD Mapper, have internal slot expanders for the many devices they carry so they can't be used on a slot expander or they have only one or two devices working when you set it to work on a sub slot.

I think slot expanders, brand new, are quite rare, occasionally someone makes a batch in Brazil, not sure about other countries. But you probably can find an used one. You can most likely have up to two slot expanders if both slots on your computer are not expanded (most likely, they aren't), but on the sub slots, you can't use those complex devices that have a internal slot expander of their own.

And a slot expander can be ordered from supersoniqs: https://shop.supersoniqs.com/en/home/80-modulon.html

1 slot to 4 expanded. in case you are interested...

the modulon? Yeah it works on msx. The way it is designed it works best in a horizontal slot. I have one myself.

The modulon has no cable. It is a cartridge that you directly insert in a cartridge slot and in it are 4 slots to put a cart (3 on the top, one in the back). Check out how your cartridgeslots are facing in your msx. If the front of a rom is pointing down the modulon design is less good for your one, but especially if you have horizontal slots it works really well. If you have a cartridge slot in the top think if it will be stable. It could very well be if you have small carts in, but big carts like msx audio or other big ones in.

There is also models of slot expanders that have 1 cart with a flatcable connected to it and that leads up to a box with 4 slots. In some cases that would work better depending on how your cart slots are positioned. But those are harder to come by.