Maybe this information will be interesting for someone of MSX-users.
I'm not really MSX-user, I more have deals with ZX Spectrum. Here, in Russia, we have many types of ZX-compatible computers, and fans of them. Some of this computers (ATM Turbo 2+, Pentagon 1024SL) are in produced by enthusiasts even today, as enthusiasts of MSX or C64 produced MoC or C-One. Additionally, some hardware upgrades are produced. In last year gained popularity sound extension called 'Turbo Sound' (TS for short). It's replacement for AY/YM sound chip - board, plugged into AY IC socket instead of AY. Board contains two AY (YM) chips, it's given six sound channels instead three, which means more polyphony, or non-overlapped sound fx in games (of course, only in games with support of TS). No additional connections with computed board required, chip selects by output in un-exists register of AY (above 15th), audio output return to corresponding pins of AY socket (also user can get output directly from TS board, if he need more cleaner sound). For this extension already exists some ZX software, include two musical editors (trackers) for ZX, and one cross-editor for win32 - Vortex Tracker II (latest versions have support for TS). Music compo for TS already spent on two our local demoparty - DiHalt'06 and CC'06.
In this year planned production of new version this sound extension - 'Turbo Sound FM' (TFM for short). It give all features of TS, fully compatible with it, and also give six channels 4-op FM-synth (OPN, two YM2203 chips on board). It sounds very close to Sega Megadrive (directly conversion of registers data from .gym or .vgm-files also possible, except digital channel and LFO features). Prototypes already were made and debugged, and production of first series must begin in first half of this year. Special music cross-editor (tracker for win32) now in developing, it already close to release, and planned to be released with TFM boards or earlier. TFM connected with computer and programming with same methods as TS.
There is no serious problems to use these two devices on MSX computers (or any systems with AY/YM chips) - except that in all devices which I saw, AY have soldered to main board (in almost all of ZX-compatible russian clones AY/YM plugs into IC socket), so desoldering chip and replacing them to socket is needed.
Of course, these devices gave nothing for old games. But that can be optionally supported in new software, and can be way to upgrade sound features of old computers without using of rare original devices.
Unfortunately (in this case), MSX already have many sound chips, but I welcome this as a oportunity for both comunities, since MSX1 and ZX have similar hardware, there's no reason why both platforms can't benefit from exchanging hardware/software. It would be nice to see more ZX-Spectrum <-> MSX hardware projects being used in both platforms, in the future. 