I managed playing music while loading from floppy!
Using this sophisticated disk drive hack: input$ in BASIC 
Problem: tested only on emu, still got to be confirmed on real machine.
The BASIC app leads to BDOS read calls of small size, 256 bytes.
Barely noticable hiccup, while a BLOAD ,S of a picture is horrors.
The BLOAD seems to call BDOS read with the full file size,
and it looks like interrupt is disabled for the whole BDOS call.
I am worried about the disk head move on file open on the real machine.
If possible, first open the file and then start the soundtrack.
Poke a quicker toggle frequency value in the test app, then hiccups get more noticable.
So, the key is to read the file in small chunks, maybe best is 256 bytes.
Then have some slooow soundtrack, so hiccups wont be so noticable.
Installation of test apps:
Make a long file named "file". It maybe better be a textfile, or loader may stop early.
Then run"fddsound.bas". It installs a simple sound toggle on &hfd9a irq handler.
Then run"fddtest.bas", the app loading the file.
if a beep stops the soundtest, type "sound 8,8" to get it back.
fddsound.bas:
10 READ A : AD = A : DEFUSR = AD
20 READ N$ : IF N$="*" GOTO 100
30 FOR I = 1 TO LEN(N$) / 2
40 A$ = LEFT$(N$,2) : N$ = MID$(N$,3)
50 POKE A,VAL("&h"+A$) : A = A + 1
60 NEXT
70 GOTO 20
100 POKE &HFD9B,AD MOD 256
110 POKE &HFD9C,INT((AD+65536!) / 256)
120 POKE &HFD9A,&HCD
130 SOUND 8,8
140 ?"change frequency by poking number of irqs in &hd028"
10000 REM**** b:dat
10010 DATA &HD000
10020 DATA 2127D0347E23BED8
10030 DATA 2B36003A29D0EEFF
10040 DATA 3229D0CA1CD0212A
10050 DATA D0C31FD0212BD03E
10060 DATA 00D3A07ED3A1C900
10070 DATA 0A0080FF
10080 DATA *
fddtest.bas:
10 clear 2000,&hc000 : on error goto 100 20 open"file" for input as #1 30 a$ = input$(255,1) 40 rem ?a$; 50 c = c + len(a$) 60 if (c >= 1024) then c = c - 1024 : ?"1k" 70 goto 30 100 if err = 55 then end 110 error(err)
Last question: use the mode command in MSX-DOS, like "MODE 80".
I don't think you can change the number of files limit just by changing a boot sector. The directory sectors won't have enough space to hold all entries...
MODE 80?! Just that?! Now I feel a real noob!! 
As for the files limit thing, you're right.
Forcing boot sector worked because I had an SD Card reared in a slot °(^_^)7
I guess its bios actually took sector 0 in account and did the magic.
Things turned to bitter normality as soon as I took it away (U_U)d
Okay, I give up.
For an undefinitely long while I won't be able to write "something smart enought" to work on 32kB MSX
thus I tighten definitions and aim to 64kB MSX.
However, I am thinking about some kind of portable format to resume this challenge later.
Besides, are there some ready-made Turbo Pascal libraries I could use to PSG and VDP?
... I'd like to know more about the plot, if there's already one.
Well, there are a couple concepts I would have used to draw a crappy comics I may use to build a crappy games instead °(^_^)d
Of course, don't expect anything original: original storilines have been exhausted at the time of Shakespeare 
Well... it's a steampunk thing which goes like this:
Temporary title: The Advenctures of Lou Vaxton, The Resonance.
The story took place in alternate 1930, more or less.
Lou works as a wireless operator onboard an italian civili airship.
After a serie of quite dramatic events, he discover a plot to build a secret weapon whose power could be used to conquer the world.
Of course, most obscure forces from all over the world plan to take over the misterious item.
After another serie of quite dramatic event and table turnings,
he will discover the real pourpose of the weapon and that some evil people is not so evil.
Well... I save you the ending
