When Town Criers ring their bells, they bellow “Oyez, oyez, oyez!” On paper it looks like “oh yes”, but it sounds like they are saying “Oh yea” or “Oh yay”. It’s pronounced that way because it is derived from, and pronounced as, Old French, with the last syllable close to the English a sound in pay. Thus whilst yes, yea and yay are synonyms, oyez is actually an imperative (an instruction to) “hear”, as in “hear ye, hear ye, hear ye,” or, in modern American English, “listen up”. It dates from the 11th century Norman conquest.
Using “yea” for “yes” has Saxon roots. It is common, but rather formal, in later (17th century) English. We find it, of course, in the King James Bible – Psalm 23, for example:
Yea, though I walk through the valley of the shadow of death…
And in Shakespeare’s Sonnet number 40:
Take all my loves, my love, yea, take them all.
It’s pronounced with that same a sound. You could argue that it’s the same as yay, but to my mind yay is a sound that comes from an enthusiastic Californian high school; yea is much more formal and pretentious: a declamation from the pulpit.
This is a bee in my bonnet because we keep finding yea in play scripts. The vast majority of the time, it is not being used with that 17th Century loftiness, but to indicate that the speaker is using a sloppy abbreviation of “yes”. We (the reviewers and editors of Lazy Bee Scripts) take the view that yea is the wrong spelling for this usage, since that spelling is already established in the biblical context. Yay would also be wrong, since that puts too much stress on the final letter, which is far more emphatic than the intended sloppy or reluctant affirmative sound.
There are two good alternatives. Firstly, there’s yeah. This shows it’s modern, unconventional English. The down-side of this usage is that there can be too much stress on the long “air” sound, making it too definite. The less definite version, closer to the teenage monosyllabic grunt, with the final sound more of a breath than a voiced phoneme, would be yeh.