The Nashville duo Hammock — celebrated by Pitchfork, NPR, the BBC, The Atlantic, and Ricky Gervais — returns on August 25 with Mysterium, the band’s eighth album since its founding in 2003. Composed as a memorial to Clark Kern, a son-like figure to Hammock co-founder Marc Byrd who died in 2016 from the tumor strain NF2, Mysterium merges modern classical, ambient, and choral music, and it’s of a piece with recent records by Max Richter and Jóhann Jóhannsson. The gorgeous, hour-long collection features contributions from the Berlin mix engineer Francesco Donadello (Winged Victory for the Sullen, Ólafur Arnalds), Peter Katis (The National, Jónsi, Interpol), and the Hamburg orchestrator Roman Vinuesa (“Iris,” “Umrika”), who transcribed scores for the Budapest Art Choir.