Here is the full list of all 45 Soundfall achievements.
Bring Brite on to the team
Deal 1000 damage-over-time with Bolero burn
Complete all missions in the Forest
Grow the team with Lydia
Defeat 100 Discordians with on-beat Audio Blade attacks
Complete all missions in the Beach
Fully slow 25 Discordians with Serenade freeze
Recruit Jaxon for the team
Complete all missions in the Skylands
Complete 10 missions with 2 or more players
Hit Discordians 100 times with Requiem chain lightning
Use gold to buy an item from the in-game store
Break down an equipment item into gold
Obtained the Audio Blade
Defeat 50 Discordians with on-beat attacks
Landed in Symphonia and completed the Tutorial
Complete all missions in the Volcano
Complete the team wth Ky
Complete all missions in the Cityscape
Faced Banshee at Harmonia
Complete all missions in the Depths
Complete all missions in the Desert
Complete all missions in the Crypts
Complete all missions in the Jungle
Complete all missions in the Peaks
Defeated Discord and saved Symphonia
Complete all missions on Dangerous Intensity or higher
Complete all missions on Extreme Intensity or higher
Complete all missions on Lethal Intensity
Complete 5 missions on Extreme Intensity or higher
Complete 5 missions on Lethal Intensity
Defeat 5 Discordians with a single on-beat Audio Bow attack
Defeat 10 Discordians simultaneously with Jaxon's Stage Dive
Hit 10 Discordians simultaneously with Brite's Area Shield shockwave
Hit 5 Discordians simultaneously with Ky's Scythe Toss
Heal 2000 health using Minuet lifesteal
Defeat 25 Discordians with Nocturne detonation
Gear up a hero with Legendary items in all equipment slots
Reach Level 50
Complete 20 missions in each environment
Complete 25 missions with each hero
Complete 10 songs using the Free Play menu
Complete all missions with a Platinum ranking on Lethal Intensity
Open 100 treasure chests
Defeat 100,000 Discordians