More informal:
* Mouth the words
* Mime the song
* Fake sing
* Dub (in the context of a recording)
More formal:
* Synchronize lips
* Mimic the vocalization
* Perform lip-synching
More specific:
* Lip-synching to a recording
* Pretend to sing (for a comedic effect)
* Overdub vocals (in the context of recording)
The best choice depends on the specific situation and your desired level of formality.