Glitch? Horn of Blasting is the only magical instrument that doesn't have "Spell Focus (Instrument)" preventing Bards from casting spells with M components. : r/CrownOfTheMagister
Horn of Blasting | D&D 5th: Demon Tower of Madness | Obsidian Portal
D&D 5e – Questionable Arcana – Crafting: Potter's Tools – Grow Up And Game