The ingame volume sliders are at maximum level. But I cannot really remember how the sound was like in the original game, because it is too long ago. Seems like I have to live with it, if the game itself is the reason. It is playable, of course, I only have to remember to turn the sound of my speakers down again after playing... ;-)
So thanks for the information!

It may have been something else, so I will tell another possibility (which may help others) :

In windows, open the "SOUND" Control Panel

In it, in the "Options" menu, check "Advanced controls".

Then, in the window you can see several more volume slides (wave, CD, PC beep, etc) : some may be lower than normal, try to push them up

Usually the most usefull one is "Wave", and for some games or programs the "Synth SW" one. Try to put both at the highest setting.

(Otherwise, even if the Main volume slide is at maximum, the sound generated via Wave or via Synth will not be at full power).

(conversely: it's usefull to push them down to really tune down some sounds without affecting everything)

Hope this helps someone out there ^^

Olivier.