When you defeat Fortissimole, his theme music becomes instrumental. You just killed Fortissimole, so he can't sing the lyrics anymore.
Aria's remix for the highest-tempo (hardest, depending on opinion) dungeon in the game, The Wight to Remain (4-3), deliberately cuts off the last note. How come? Aria dies instantly when the music ends and can never hear the last note.
Aria didn't use the Lute herself because she doubted her own spirit. Melody could allow some playing relatively safely, Aria felt she was too weak (due to both her weaker physical state and playing the Lute a lot before) and could just end up being a second Necrodancer. Although she probably plays the Lute a little between floors to not bleed out completely.
Why Melody has to play constantly while Aria doesn't? She is unexperienced with the Lute and can not make herself alive permanently; that is done later by Aria when she comes back to life. That is also why Melody doesn't die once the Lute is destroyed.
Eli's default sountrack is a Chiptune remix, after unlocking Dorian in the DLC Amplified, he also has a Chiptune remix by default, why? Because they're brothers.
Why don't life-saving items like the Potion stop Monk from dying when he touches gold? Because he's failed his own vow, so he's inflicting the damage to himself and wouldn't save himself!
