Here is the full list of all 16 Songs of Silence achievements.
Finish campaign map 1 "The Siege of Elbin"
Finish campaign map 2 "Into the Silence"
Finish campaign map 3 "Songs of the Blue Vale"
Finish campaign map 4 "Dark Tidings"
Finish campaign map 1 "The Siege of Elbin" with all "Ehrengard Refugee" units still alive
Complete Pip's quest on campaign map 1 "The Siege of Elbin"
Speak with King Atran after saving him on campaign map 1 "The Siege of Elbin"
Finish the adventure of "Balmung Forest" in campaign map 2 "Into the Silence"
Rest at the statue of Saint Idha
Escort all "Ehrengard Refugee" units safely to New Ehrengard
Save Freymund in campaign map 3 "Songs of the Blue Vale"
Finish campaign map 3 "Songs of the Blue Vale" in 22 turns or less
Find Hauser's secret meditation spot
Befriend the Forlorn on campaign map 4 "Dark Tidings"
Fight against Garin on campaign map 4 "Dark Tidings"
Win a skirmish or multiplayer map against at least 2 enemy factions without losing a single battle