Fullmetal Alchemist - FMA where everything is the same except Al is Weird Al by Morbi

I know both.

Senior, Weird Al Yankovic is a singer that does parodies of songs. Examples of his works include a parody of Michael Jackson's Beat It but titled Eat It and a parody of Queen's Another One Bites the Dust titled Another One Rides the Bus.

Toy Soldier, Fullmetal Alchemist is an anime involving alchemy, changing one form of object into another, so long as the mass remains the same, otherwise known as equivalent exchange. In the case here, Edward Elric performed human transmutation, losing his brother, called Alphonse Elric's body in the process, so performs another transmutation onto a suit of armour which brings his brother's soul into the armour, making a sentient suit of armour.

Put them together, and you have Edward Elric performing his transmutation on an accordion, which results in Al being brought back and making crazy song parodies every time he speaks.