Avicii likes mixing EDM (electronic dance music) with all kinds of other genres.  On 'Wake Me Up' there was a tinge of country/bluegrass, but on 'Hey Brother' it is full out bluegrass inspired.

The vocals are by American bluegrass singer, Dan Tyminski.

Dan Tyminski Facebook

Dan Tyminski is a member of Union Station.  You may know Union Station as the band that backs Alison Krauss.

Dan Tyminski Facebook

Dan Tyminski was also the featured vocalist on the revamped version of 'Man of Constant Sorrow' from the movie 'Oh, Brother Where Art Thou?'