Sound of ea

Original pronunciation

They had a long e sound in middle English.

Changes

Long i

Later, during Great Vowel Shift most ea words became long i.

DRESS e

Some ea sound like DRESS vowel e before certain consonants (like d, th, l)

ei

There is no productive rules regarding this. They are fixed lexical exceptions.

i-schwa (Traditional British)

This is Centering Diphthong

The tongue moves towards the central vowel schwa, as they originally had e pronunciation - but in non-rhotic accents, they became centering diphthong.