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I can't say for sure, but some care providers feel strongly that relaxing your throat and face helps you relax the perineum and birth more efficiently. Ina May's books discuss this a lot. A long, low, "moooooooo" type vocalization will open up and relax the face/throat muscles and may help the birth proceed more smoothly. So this could be what your midwife was asking you to do.
Others feel strongly that if you are vocalizing a lot in the upper register then you're "wasting" energy that could be used for bearing down. Think of it kind of like weight lifting... that 'grunt/ungh' sound they make as they lift? That's a "lower voice" sort of sound in that context and might be what your midwife was thinking about.
Either way, I agree that there are births where no amount of "should" is going to matter. I "mooooed" my way through a very long labor that ended in a c/s. I hypnobirthed and breathed and grunted my way through a vbac. And I screamed like a banshee for the full four hours of my second vbac (to the extent that my midwife left orders for throat lozenges

). Each birth prompted a different vocalization response.
I'm guessing your midwife thought that if you changed how you were vocalizing, the birth might have gone faster or more smoothly or something like that. Sort of like suggesting a position change? But in the end you're the one having the baby and you're the one who is ultimately in charge.