Final edit :
I found the issue, here's the description of what I did and how I fixed it, should someone have a similar issue one day.
• I made a copy of my most recent file, gave it a try : no sound of course.
• I started to trim my project file down, removing things step by step, and checking regulary if something would work. I started to remove all the characters but the main one, I then removed all the different scenes and locations, I removed all the actions, conditions, sounds but my F10 one.
Finally, I ended leaving only the blank title screen with an EXIT button. Everything else was gone, no more Start actions, no more particules, no more interfaces but the mouse.
I hit F10.
• I then went to my Game Properties and gave it another look again. I tried some of the options here, changed the action text, etc, etc...
• Finally... I erased two elements : Company name and Game name.
F10 : and here was my sound !
I went back to the Game Properties, wrote down the exact same information again... and... the sound was gone again.
I erased the information : sound was back.
I noticed that, when I left the mouse cursor over the Company & Game Name parts, Visionaire displayed briefly a huge block of help text (which sadly goes away way too fast everytime, so it's a bit annoying to read it XD)
And here I read that Company Name and Game Name were used to save some important data of the game in the Local %appdata% folder.
So I went there... and... indeed.
I found a "config.ini" file, opened it, and two of those volumes were set to 0.
So I erased my Company & Game Name folders from my %appdata% folder, launched the game anew, tried if the sound was still working, which it was, I then went back to check the "config.ini", and this time the sound wasn't set to 0 anymore.
(but I'm still a bit confused, cause I still have GlobalVolume = 0 in there, but sounds works well, is that normal ?)
I tried to understand what happened.
And yesterday, during my final steps to finish my demo, I toyed around with an Action to control the audio volume : "Change Volume".
But as I didn't get it to work, I removed it again from my project file.
But it seems that this single test I did modified the whole "config.ini" and didn't switch it back once I removed the Action again.
And this also explains why my 3 weeks old .ved file still worked : cause I didn't write Game & Company Names down at that moment, thus, it used the default folder, which wasn't modified by my aborted audio test I did yesterday.
Long explanation, sorry for the annoyance
Thanks again for your help ! ^^
(But no worries, I still have 1 or 2 questions left, so I'll surely annoy you some more in a different thread