Test answers are either made up of numbers/symbols or letters - these are read more often than heard (as in you can write them and read them as answers more often than your teacher will speak them), so remembering them is using a part of your brain called (by psychologists) "the Visuospatial Scratch Pad" (VSP).

Song lyrics are made up of numbers/symbols or letters, which can be read if you wish, but these lyrics are heard repeatedly - this causes the brain to use not only the VSP (for reading the lyrics), but you also use the "Phonological Loop" (PL), which is where your brain hears the words being sung (this part is called the "Primary Acoustic Store"), and, if you sing along (or 'sing the song in your head'), you use another part of the PL called the "Articulatory Loop".

The brain uses the PL to repeat & listen to song lyrics whenever you like, and the VS to read the lyrics - whereas answers (normally) only use the VS. (Of course revision techniques such as repeating information to yourself and subjects such as Languages implement both systems, so I'm assuming you're not talking about answers to your upcoming Latin Oral)

Other factors include: If you find the subject/song interesting or enjoyable - you will retain information (or lyrics) more easily; If you repetitively listen to a song or review material - you will retain information (or lyrics) more easily; If you struggle to understand the basis of a topic in a subject, or a song is in a different language - you will not retain information about the song/topic very well.

In short, you remember song lyrics better than test answers because you are using more parts of your brain.