Earpieces of Tranquil Focus
Source: Other. Can be purchased from the wondering Gnome with a cart () in Stormwind or the wondering Goblin with a cart in Orgrimmar () for 45g - 50g (Thanks to stevethepotato for the info on the horde NPC).
The Earpieces of Tranquil Focus are a vendor toy from Classic that mutes certain sound effects around you and can be purchased from wandering cart vendors in major cities.
Guide awaiting review
Locate the vendor
Find the wandering Gnome with a cart in Stormwind (Alliance) or the wandering Goblin with a cart in Orgrimmar (Horde).
Purchase the earpieces
Buy the Earpieces of Tranquil Focus from the vendor for 
45-50 Gold.
Tips
The earpieces primarily block out voice lines from players transformed by the Transmorpher Beacon, excluding voice lines from your own character.
They can mute musical and vocal sound effects from toys like
Toy Train Set, 
Seafarer's Slidewhistle, and 
Darkmoon Whistle.
