Put an audio file back in the map folder, make sure its named the same as it was before. It was most likely audio.mp3, if not for some reason, check the .osu text file of your map, it should have the name of the audio file in there near the top.
Then, you have to let your client refresh the map if you had it open, so leave the editor and highlight a different map so that its song plays, then you can open your map again.