I tend to turn the light up (where possible) about a minute before the last song has finished and turn my disco lights off then, together with making it 'obvious' it is the very last song using the mic.
Never thought about putting background music on, but surely you will just get people asking for it to be turned up or making requests ?
I never play one more as always ask the B&G to pick a song to finish on and make sure everyone knows that the final song is one specially chosen by the B&G.
I think playing background music could create further problems - when do you actually turn it off for one ?