There are two types of songs in the Sing! app:
Songs in the official songbook: Those songs were produced by Smule after negotiating the rights with the rights owners of these songs. You can recognize these songs by the Smule logo next them. These songs have an excellent quality and they always have accurate lyrics and a cue track for the melody.
Songs in the community songbook: Community songs were uploaded by users of the app. These could be songs the users recorded themselves or karaoke tracks they got from YouTube or Karaoke CDs for example. Or they might be derived from the original songs with attempts to mute the voice of the artist. The quality of these songs will vary. The lyrics might be wrong or missing or the timing might have problems. The (automatic) cue track might also have problems. And there is always the chance that these songs (and with that all solos and collabs created with them) will be removed, either because there were copyright claims or because the user who uploaded them deleted his/her account.
Official song (top line) and community song (bottom line)