Though my French is really too crap for holding a proper conversation.

what? Are you joking? As far as I've seen, you have a very decent French.

There are talented programmers on system-cfg.com, you'll see

He loaded it twice because the hector is faster than zx spectrum: "Comme Hector est plus rapide que le ZX Spectrum ( de 3,5 Mhz on passe à 5 !) cela suffit à descendre le tempo (c'est pour ça qu'il y a 2 fois le même ld)."
It was also a "quick and dirty" conversion, he said he hasn't optimised the cycles used in your engine for getting a correct timing, hence the noise.

   ld (Save_RegA),a
   ld a,_LOW
   ld ($1800),a         ;du son à Hector !
   ld ($1800),a         ;du son à Hector !
   ld a,(Save_RegA) 

should at least be

   ld (Save_RegA),a
   ld a,_LOW
   ld ($1800),a         ;du son à Hector ! - why twice? once should be enough?
Save_RegA equ $+1
   ld a,0

and since BC is unused, in most cases even

   ld c,_LOW         ;outside of sound loop
   ;...
   ld b,a
   ld a,c
   ld ($1800),a         ;du son à Hector ! - why twice? once should be enough?
Save_RegA equ $+1
   ld a,b

should do.

system-cfg.com seems like a nice place, I'll think about signing up. Though my French is really too crap for holding a proper conversation.