From U2.com:
"The remastered Joshua Tree features several unreleased demos from The
Joshua Tree Sessions. Bono recently added new vocals to one of them,
'Wave of Sorrow’, and this week talked about the inspiration for the
song. Watch our exclusive video.
Interviewed in New York, Bono explains that Wave of Sorrow ‘was one of
the songs from the Joshua Tree sessions that we never finished… a song
that was trying to describe experiences that myself and Ali had when we
were working in Ethiopia during the famine.’
The track is one of several rare and previously unreleased cuts which
feature on the bonus audio CD, which comes with the deluxe edition of
the re-released album."