The words moan, mown sound the same but have different meanings and spellings. Why do moan, mown sound the same even though they are completely different words?
The answer is simple: moan, mown are homophones of the English language.
A low, sustained, mournful cry, usually indicative of sorrow or pain.
A similar sound: the eerie moan of the night wind.
To utter a moan or moans.
Past participle of mow
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.