The Musica Infernus can be found on the second floor of the Clothing Store and purchased for 45 Bucks. Purchasing this hat will allow you to wear this hat only in Adopt Me!, unless you buy this item from the Roblox Catalog.
The description of this item on the Roblox Catalog reads
"'Musica delenit bestiam feram.' This item came out of the Sounds as a Bell Gift on December 17th, 2013."