Update:

Ok, while almost giving up, I tried with the SCD-15/SCB-55 today, just for kicks - and that works perfectly!
Hmm, I think, let's try that SCD-10/SCB-7 again... wow! Now that suddlenly works just fine, with all the songs as well.
Oh joy, right ? :\

Ok, now for the $64 million dollar question... with all this sudden success since yesterday (remember, I haven't changed a single bit of the rest of the setup, hardware or software settings) I wanna go for gold with the Yamaha DB50XG... that would now miraculously work as well, correct ?

Monkey Island 2 theme, just dandy, no machine guns there...!!
- Super, I'm golden!!

Then, just to make sure, Star Trail 1.... OUCH OUCH OUCH!.... same crap as earlier... oh well, must be this particular song only then, right ?

I then replay the Monkey Island 2 - and NOW it goes back to bananas again !!?
What gives ?

With my puny knowledge of any of this techie/music/midi stuff, I wonder, is there something in that StarTrail song that just fubars some settings somewhere ? Something left in memory ? Patches getting the wrong numbers etc ? Rolands way of dealing with the competition ?

- so, I'm back to square one here... ack. 😵

More testing: I restart the machine, with the Yamaha still in.
Other than having to reboot again before win98 kicks in, due to the idiot OS(?) claiming there is no himem.sys to be found (I find it easily using DIR commands!), I try again, Monkey Island 2 as the very first song.

Perfect sound! Hmm, something must have gotten flushed during that reboot ? But WHAT ?

Now StarTrail again - as crappy as usual.... and then it's back to that square you know so well for the rest... 😢

Theory:
Maybe that song uses some GS parts (I'm not really sure if it is a GM or GS file), and the Monkey Island2 is only GM ? And being a Yamaha card it's different from Roland GS, and hence messes things up, when then trying to play plain GM again ? But why on earth does it do this ?
Though it's patches or whatever may be somewhat different, it certainly shouldn't sound like that in the first place I think.

The 2 Roland daughters were just happy alternating between the files, so...

- OR, Is it the MS Mediaplayer 6.4(standard version) that is to blame ?!
Which is the 'test program' I've been using so far...

Further investigatons: I have some Roland related software I found, SongCanvas shareware from 1993(!), which plays the StarTrail song just beautifully...
(while we're still! playing on the Yamaha, where Mplayer mess up)

'Funny thing' now is, that when I try the Mplayer again after messing with the SongCanvas program, I suddenly get NO sound at all - on any midi file.
Obviously there are happening things to my computer out of my control...

...hhHeeelp !!! ?

- Boy, it's hard to work forensics while clueless, I can tell you.

***

Ok, I'll take a long break now, and a cold shower.... 😒

Sorry for the ranting.