"婴儿发出的咿哑声" can be translated into English as "the babbling sounds made by a baby" or simply "a baby's babbling".
"Babbling" specifically refers to the meaningless but rhythmic sounds that infants make as they practice vocalizing, which captures the essence of the "咿哑声" (yīyǎ shēng) described in Chinese.
The phrase "made by a baby" or the possessive form "a baby's" clarifies the source of these sounds.