"单调的吟唱" can be translated into English as "monotonous chanting" or "monotonous intonation" (depending on the context, with "chanting" being more literal for the act of singing or reciting in a repetitive manner, and "intonation" focusing on the tone or melody being单调/unvarying).
Monotonous chanting emphasizes the repetitive and unchanging nature of the singing or reciting.
Monotonous intonation could be used if the focus is more on the tone or melody lacking variety.
In most cases, "monotonous chanting" would be the more straightforward and appropriate translation.