This is something that confuses a lot of native speakers. It seems Romanians use both. This article tries to make sense of it, basically claiming that the plural is used when the emphasis is on the individuals and the singular is used when the emphasis is on the group. Indeed, this seems to cover how speakers use singular/plural intuitively (based on my own observations).
I think "ea" is a dipthong so it is only one vowel sound. (Not e-a, but ya.)
You say it is not left out, but in the recordings, ea sounds like only one vowel sound, an a. Does the e simply palatalize or soften the preceding consonant, as final i usually does?