The original choral work “Sleep” was set to Robert Frost’s ‘Stopping By Woods on a Snowy Evening.’ Then came the legal tussle and Eric Whitacre simply rewrote the text via a good friend. He still created a great piece (see video). However, the big point here is:
The original poem was published in 1923. In a sane copyright regime, it would be public domain already. (Granting from death of author is just a bizarre notion).