From Longman Dictionary of Contemporary Englishsing along phrasal verbSINGto sing with someone else who is already singing Sing along if you know the words. to Jess was singing along to the radio. → sing→ See Verb table
Examples from the Corpus
sing to• I like things in the car to sing along to.• He sings along to a recorded guitar background.• I like songs that you can sing along to, and you can remember.• I was singing along to the Corrs quite happily when I noticed a group of people looking in my direction.• Jackie was singing along to the radio and didn't hear the doorbell.• You can also sing along to the songs if you wish. 