Why the "sing-song" voice during the prayers?
That would be Gregorian Chant, a traditional form dating back to times before printing presses and mass literacy. The rhythmic cadence helps with memorization, as well as setting a solemn tone for worship. It is for the benefit of the faithful to help them follow the familiar sequence and thus pay attention to the mass, not to help God "hear better".
I get "why" it's probably done, but "whew" what a time consumer.
The "Alleluias" precede and follow the Gospel Antiphon, a Psalm passage selected to introduce the Gospel to follow (it relates to each day's Gospel reading). It also sets a reverent tone appropriate for hearing the Gospel, which is the highlight of the "Liturgy of the Word" (not merely a footnote, as it would be in a Kingdom Hall "talk"). Singing or saying the antiphon takes maybe a minute, hardly a big time consumer.