How to pronounce えい and おう

I’ve been learning Japanese for around 3/4 of a year now, and I still don’t understand how you’re supposed to do it.

I often hear えい the way you’d expect it, but sometimes I hear it pronounced as ええ. Same for おう which sometimes gets pronounced おお.

I’m definitely not hearing wrong, so can someone please explain how I’m supposed to pronounce them (in which case)? Thanks in advance