i found a solution which works fine and wasn't such much work
to do, i thought before...
the first call goes to a frameset, where one frame is filled
with the startpage and the other frame (just 1row and color to
background #fff, so it's not visible - don't set the visibility to
hidden, then you get no sound too!).
into the second frame i put the .swf file (width and heigth
=1 px), with the looped sound, which now playes in all pages and
even in the pageloading times...
if nesseccary you could also add buttons to controll the
sound, but they are outside the main window (frame 1) of your site.