The words sewn, sone, sown sound the same but have different meanings and spellings. Why do sewn, sone, sown sound the same even though they are completely different words?
The answer is simple: sewn, sone, sown are homophones of the English language.
A past participle of sew.
A subjective unit of loudness, as perceived by a person with normal hearing, equal to the loudness of a pure tone having a frequency of 1,000 hertz at 40 decibels.
A past participle of sow1.
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").