According to merriam webster online:
1. a lyric composition; specifically : a lyric poem (with songlike quality)
2 : the words of a song -- often used in plural (lyrics)
So, you have answered your own question. "Heavy metal thunder" is referred to as a lyric poem (a song). If you refer to its content, or words, then you refer to the song's lyrics. (What are the lyrics of this song?)
the lyric is...
the lyrics are...