In other words, if the music infringes on another work, the entire song (including lyrics) is liable.
Well, people who sue for infringement will claim they then own some share in the infringing song's profits.
What part of the song is infringing won't matter, so it could be lyrics, melody, a bassline... whatever.
How
much of the profits they feel they are due depends on different factors, but I
supect it's more a function of "how much to we think we can get" rather than some actual calculation.