Oh ... heh heh heh ...
A) I uninstalled the program (still unending music/background sounds)
B) Restarted system *** Music/Background sounds GONE/Stopped!!! ***
C) Reopened Firefox (Phew! Still no music/background sounds)
D) Restarted Previous Session - all Firefox tabs restarted *** AND SO DID THE Music/Background sounds! ***
E) Closed the 2 Machinarium website tabs *** Music/Background sounds STOPPED Immediately! ***
DUH ... and Phew!