In my screenplay, the lyrics of "Shenandoah", a traditional American folk song, are heard once. May I publish a few lines with the words?
Assuming you mean "Shenandoah", that one seems safe enough from a legal standpoint, being quite old and a folk song with no clear author; but be careful with more recent works. A work can be in the public domain federally but still be under copyright in some states or encumbered in other ways. Nina Paley was famously bitten by this when she made Sita Sings the Blues. And, of course, international copyrights might apply to anything.