The words muscle, mussel sound the same but have different meanings and spellings. Why do muscle, mussel sound the same even though they are completely different words?
The answer is simple: muscle, mussel are homophones of the English language.
A tissue composed of fibers capable of contracting to effect bodily movement.
A contractile organ consisting of a special bundle of muscle tissue, which moves a particular bone, part, or substance of the body: the heart muscle; the muscles of the arm.
Muscular strength: enough muscle to be a high jumper.
Informal Power or authority: put some muscle into law enforcement.
Any of several marine bivalve mollusks, especially the edible members of the family Mytilidae and in particular Mytilus edulis, a blue-black species raised commercially in Europe. Mussels are often found attached to rocky surfaces or the sides of ships.
Any of several freshwater bivalve mollusks of the genera Anodonta and Unio, found in the central United States, that burrow in the sand or mud of lakes and streams.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").