Yes, I agree that Fez isn't "about" a child being born. However, the lyrics clearly describing childbirth (
A speeding head, a speeding heart /I'm being born,
a bleeding start/ The engines roar,
blood curling wail /
Head first, then foot / The heart sets sail) IMO are meant to lay the physical, obvious groundwork for the more esoteric, spiritual meaning. Metaphor, man, metaphor.
Thus, if the sexual meaning of "let me in the sound" is indeed real and intentional, then it's possible that it's used as a physical base for the metaphor, as well.