While Hans clearly has it, I'm still working, slowly. :) Clues made me look at the Vienna Opera House initially, but too far from the river and none of the nearby bridges look right. Nonetheless, a beautiful and wonderful-sounding venue! A quick survey of other bridges on the Danube didn't turn up likely matches for me, but others may have more luck - jump in! (But don't jump off a bridge in Paris... you'd be in Seine...)