I've seen both being used interchangeably. Are both valid? Is only one correct?
- Lyrics to a song.
- Lyrics of a song.
This Google NGram indicates that both are in use, with "lyrics of a song" being more popular:
This phrasing may be more popular because "lyrics" is a property of a "song". That is a song has lyrics, so the lyrics are of a song because this is the possessive form. I think that both are sufficiently valid (as NGrams records use from a corpus of written material, which tends to be slightly more grammar-conscious than online works such as a blog), so you could use either and be understood.
This is personal feeling, but I'd just like to say that:
"Lyrics of a song" is more correct. We usually say "words of a book", "works of an author", etc. That's because you're asking for something that belongs to another, therefore "of" should be used.
"Lyrics to a song" is correct in its own way, but shouldn't be used when asking for the words of a song. "Lyrics to a song" should only be used when referring to someone giving words to a song:
That's personal opinion, and I don't really have any sources or links or rules to back me up. Maybe someone else has.