I think it depends on the person. As a kid, I learned that those (les, des, mes) were supposed to be pronounced "è", but in normal conversation I always hear them pronounced "é". Maybe it's a regional thing. Curious to hear what others will say! :)

E alone is "uh" ES is exactly the same as "è" (like the English E in the word "jet" or "message") É is "ay" (like the American A in the word "game")