sing along
To sing the same words or music of a song at the same time as it is being sung, performed, or played. He encouraged the audience to sing along, but only a few people participated. Our toddler always likes to sing along with us whenever we start belting out some Disney tune or another. I think it's cute the way he sings along to the radio in the car.
sing along (with someone or something)
to sing with someone or with the accompaniment of some instrument(s). Harry played all the old songs and everybody sang along. Let's sing along with Mary. She knows some good songs.
sing along
v. To sing in unison with a song or singer that one is listening to: The performer urged the audience to sing along. I turned up the radio and sang along to the song.