The words base, bass sound the same but have different meanings and spellings. Why do base, bass sound the same even though they are completely different words?
The answer is simple: base, bass are homophones of the English language.
A line used as a reference for measurement or computations.
The lowest or bottom part: the base of a cliff; the base of a lamp.
Biology The part of a plant or animal organ that is nearest to its point of attachment.
Biology The point of attachment of such an organ.
A singer who has such a voice.
Any of several North American freshwater fishes of the family Centrarchidae, related to but larger than the sunfishes.
Any of various marine fishes of the family Serranidae, such as the sea bass and the striped bass.
A low-pitched sound or tone.
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").