yay / yea are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
yay: (noun) a branch of the Tai languages.
yea: (noun) an affirmative. (adverb) not only so, but.
yay: y·EY
yea: y·EY
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)