The words mussed, must sound the same but have different meanings and spellings. Why do mussed, must sound the same even though they are completely different words?
The answer is simple: mussed, must are homophones of the English language.
Simple past tense and past participle of muss.
Used to indicate inevitability or certainty: We all must die.
To be obliged or required by morality, law, or custom: Citizens must register in order to vote.
To be compelled, as by a physical necessity or requirement: Plants must have oxygen in order to live.
Used to express a command or admonition: You must not go there alone. You simply must be careful.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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").