DREAM

mp3 is finally playing under Windows but not yet a full victory...

Even though I kept repeating myself we didn't have to look for modifying the code,
finally the Opera engineers themselves confirmed it was a plug-in issue.
As a matter of fact (two weeks later) the tune is finally playing on the Windows platform.
Don't tell me this had to take 'that' long...

BUT
We're not quite there yet!
Yes, the tune is playing with a simple audio/mpeg attribute triggering the QuickTime7 player to kick in gear.
(It started doing that after copying a dll in the installation directory and pasting it in the Opera plug-in directory.)

So,

<embed src="../../rootDirectory/subDirectory/tuneTest.mp3" width="140" height="17" autostart="true" loop="FALSE" type="audio/mpeg" wmode="transparent"> 
</embed>

does the job up to the point of auto-playing the tune without controls being displayed when triggered by XP Pro on the Windows platform. This is an utmost strange behavior of the 10.10 browser since that same browser running on the OSX platform DOES show the player controls.

Do you get it? I don't!

But there's hope. A final solution to this problem can't be far away anymore.

Tx to all at Opera for assisting while trying to solve this problem.