"Five" was the first map which allowed players to play as popular real-life historical figures. It also ended up as the first map to use a Real Song Theme Tune thanks to Eminem. Call of the Dead - which instead features real-life actors like Sarah Michelle Gellar and Michael Rooker - would follow suit with Avenged Sevenfold's "Not Ready To Die" which, unlike "Won't Back Down", is an original composition made for the map itself.
As some keen listeners on YouTube pointed out, "Archangel" has a BPM of 230. 230 divided by 2? 115! Oh, and there's plenty of callbacks to previous Easter egg songs, such as "The One" (Waiting for the one), "115" (I'll bring them down on my own), "Pareidolia" (So painfully and slow), "Coming Home" (I'm coming home), and "Always Running" (I'm becoming one).
Though it technically debuted in World at War, Nacht Der Untoten - which is the first Zombies map as a whole as well as the beginning of the Aether story - wouldn't get an Easter egg song of its own until the first Black Ops, being "Undone". Tag Der Toten - the very last map of the Aether story - features "A Light From The Shore", another instrumental piece that, as some YouTube comments pointed out, feels more like the music you'd hear during the End Credits. All's well that ends well, right?
Then there's also the fact that "Nacht Der Untoten" and "Tag Der Toten" are German for "Night of the Undead" and "Day of the Dead", respectively. Enough said.
On top of all the horror of Mob of the Dead and realizing that the characters have been stuck in this cycle for who knows how long, the fact that "THE CYCLE CONTINUES" if Sal, Billy, and Finn kill Weasel on the bridge brings up another terrible possibility. Since Mob restarts from what we saw at the start of the match if the cycle continues, it's completely possible that the mobsters have already come this far before. The climactic clash on the bridge could've happened an infinite amount of times, with the crew remembering their fates in real life just like they do when you control them, but they still chose the bad path nonetheless. This easily could be far from the first time that the events of Mob up to the end of the egg have happened.
. Tag Der Toten - the very last map of the Aether story - features "A Light From The Shore"