When listening to Itachi's theme song, I heard that there is a woman voice in it, however I can't understand what it is saying. What is the woman saying in Itachi's theme song?
From Google, I found that Itachi's theme song is titled Senya (千夜, lit. Thousand Nights) and is part of Naruto Shippuuden Original Soundtrack 2. This song has no lyric. The woman voice there is part of the instrument. If you listen to it carefully, the woman only voice the vocal "a".
According to learntoplaymusic.com
the human voice can be thought of as the ultimate melodic instrument, because it is capable of instant expression with no instrument required to translate thoughts and feelings into sound.
The voice is there to give more feelings to the music about Itachi's pain, which just instruments won't be able to deliver.