There are 16 lyrical songs produced by Treyarch Sound for the Easter Eggs in the Zombies storyline. There are also four playable characters. That evenly divides the songs into four groups. While it might be... pareidolia... to mark this... I don't think it is. I found a few interesting connections when separating the songs into four distinct groups.
The first group includes all of the music from Call of Duty: WAW Zombies, and one song from Call of Duty: Black Ops Zombies. They are Edward's inner thoughts and conversations with the other four.
Lullaby for a Deadman - Edward Richtofen's thoughts about himself, including parts of what he thinks about the other three.
The One - Edward explaining to Takeo what happened in The Giant, from the point of view of a Richtofen 1.0 who did NOT get killed by a Richtofen 2.0
Beauty of Annihilation - Edward explaining to Nikolai his anxiety, paranoia, and delusions, as an excuse for his atrocious acts.
115 - Edward's attempt to explain to Tank that he (Edward) thinks that god is dead, and that's why he's an Atheist, not because he feels that God doesn't exist.
The first three are in fact from the same game, but 115 is from Black Ops. Originally, Kino Der Toten, the map that 115 is part of, was meant to be in WAW, however, they ran out of time before it was complete, which means, less than ironically, that 115 actually DOES belong in WAW.
The next four songs are the inner thoughts of Takeo Masaki, and conversations he had with the other three. They include three songs from Black Ops, and one from Black Ops II (the only reason a BOII song is here is because they had to move one to BOII because of Kino being on BO)
Abracadavre - Takeo telling Richtofen, "If you need me to be the One to kill you, I will do it." as a response to the conversation Edward and he had about the events of The Giant.
Pareidolia - Takeo's inner thoughts and understandings about "The One", concluding that if he is NOT the one, then the realization will be long, drawn out, and painful, based on the fact that he truly thinks he is the One.
Coming Home - This is a conversation Takeo and Nikolai had, regarding "The One", and if it would be better to let the One kill Edward, or to sacrifice the One in the hopes of having a better future.
Where Are We Going - This is a conversation Takeo and Tank had, regarding whether or not they were actually accomplishing anything with their tasks and with following Richtofen. Takeo doesn't talk to Tank about the One, based on the fact that Takeo doesn't feel that Tank could be the One. He hints at the concept by talking about "The Calling", and ponders on the ramifications of taking matters into his own hands.
The next set is the rest of the songs from BOII. There is an extra, here, and it aligns perfectly with the fact that Origins, honestly, should have been in BOIII. It should have been the first map. They did a reversal of what happened regarding Kino Der Toten, and sacrificed the quality of Origins, to make up for the extra quality of Kino Der Toten. Was the sacrifice worth it? I don't know.
We All Fall Down - This is Nikolai talking to Richtofen about the fact that Edward NOT dying in The Giant in his personal timeline is an error. Their death made a future that could be enjoyed and loved by everyone, and the reality of their actions led to untold chaos and destruction. He's trying to comfort Edward in the hopes that Edward will correct the "Martyr's brethren being set free", and the fact that the correct chain of events never happened. Edward is terrified of the Shadowman, which we now know was one of the voices in his head, thanks to the timeline that Treyarch released.
Always Running - This is Nikolai explaining to Takeo that no matter what choices they made, that their end was the same, regardless. They all died, and the very thing they were running from would be the tool of their demise... that they would, too, rise, and attack the living, and one day, someone would end them, the same way they ended countless zombies, themselves.
Carrion - This is Nikolai's thoughts regarding his hatred of all living things, recorded in his Bio in WAW.
Archangel - This is Nikolai warning Tank of the dangers of messing with Richtofen, that Richtofen only aims to control him to prevent his own demise. Tank is the only member of the team that actually listens to Nikolai, and so if there was one person Nikolai didn't hate, it would be Tank. He's the only one Nikolai would share his vodka with, and the only one he would follow if his memories were erased (if he ran out of Vodka, like he did before Call of the Dead). Nikolai is useless to Richtofen without his memory, because Nikolai without his memory hates Richtofen even more. At least when he has his memories, he knows Richtofen knows what he's doing.
The next set is the songs on BOIII, and they detail Tank's personal thoughts, feelings, and conversations with the other four. One that truly understands them realizes how much pain the writer must have felt in writing them. They deserve to be understood.
Dead Again - This is Tank talking to Richtofen, so far, he and Richtofen are the only one's that are dead, so Edward is his only company. Only now is Tank willing to actually follow Richtofen. It's a reversal of his 2.0 self.
Dead Flowers - This is Tank talking to Takeo, mostly about the realization that nothing they did really mattered, when all they did was kill. They couldn't cure, and the end result of their strife was death for them, nonetheless. The joy he once found in the violence is gone, and all he hears is the pain he causes. Tank no longer enjoys his lot in life.
Dead Ended - This is Tank talking to Nikolai, hoping for a way out of the endless cycle they're in, Nikolai warns Tank that in order to destroy the cycle, they have to create a new one as well, or there will be suffering for everyone. Tank doesn't care about the danger, and feels that the mission in mind is more important than the risk.
The Gift - This is Tank's personal thoughts towards himself. He thinks about everything he gained, everything he lost, all of the times he's killed, and whether or not it's worth it. He realizes that the people he loves have been hurt by him, and he is basically saying to himself that that won't be his only legacy.
WAW = Herzog De Per'me (Duke of Parma)
BO = Faust
BO1 = Gretchen
BOIII = Erzengel (Archangel)
Shadows of Evil = Mephistopholes
Zombies Chronicles = Faust (the second one)