1. In a bunker behind the Aqua house is a bunker, inside the bunker is Marlton (Guy with glasses) from Tranzit. You have to knife the bunker to hear him, this proves that Nuketown must pre-date Tranzit and that he must've took the 2nd bus route back to Green Run.
2. Throughout the rounds you can Richtofens voice from a radio inside the Aqua house, you can hear his voice every 5 rounds and these are the quotes (in order)...
..."I knew I should've invented an egg moving robot. Stupid Maxis and his rules, and his stupid accent!"
"Login, Teddy. Password, is a liar. I know but that's why no one would guess it."
..."Hello Samantha, you little brat. Your time is coming... haha... soon!"
"This is going to be so much fun! Finally we can confront this little beast, quickly power up the machine."
..."THE POWER, THE POWER! I can control them all, I will destroy all of you! But not before we finish the game."
The last one can be heard at Round 25 and after you hear that you will probably notice that the zombies eyes are blue instead of yellow, meaning that Richtofen is controlling the zombies. Also if you're a hardcore zombies player you would probably know all the quotes are from Moon. This is where Richtofen and Samantha switch souls.
3. At round 3 or 4 you can hear the Green Run bus which is the 2nd bus that goes to the alternate route and the bus that Marlton must've took to escape the nuketown.
4. This next one is a hidden song, to do this you must have the clock at 1 and the population counter at 15, when put together is 115 (element 115). Now you must buy the door in the back of the yellow house that has a power-up, once it's picked up a song will start.
5. Now there are 2 songs left, all can be easily activated. The first one is by activating all of the teddy bears and the second one can be by shooting all the heads of the manequin.
At the end of the game you could also see a missile about to hit Nuketown, my theory is that since this is going on at the same time as Moon it must be the missiles that are launched after the Big Bang Theory.
[Update #1] (1/25)
As many of you know Marlton can be heard inside the the vault, here are his correct recordings.
"Visibility here is zero, what's the point?"
"This device will prove my mental superiority once and for all."
"I cannot believe this is the best route to where we are going."
"The hydronic forces contained herein shall make me unstoppable."
"No! My electronics, ruined!"
"The air is thick with the sounds of carniverous beasts."
"They ask me to leave? I was the one reporting this scuffing behavior... makes no sense.
"Execute them before they overwhelm us."
"Just because one is different is not a valid excuse to simply banish him."
"Ahhh the amperes(amps) have hurt me to the quick."
"Oooh what an interminable wait."
"Please respect my personal space."
"Grow a pair of damn eyes already there are zombies on the roof.
The last quote might help solve some other riddles but I have yet to find out that right now, leave a comment and tell me what you think it means. Leave a thanks!
I hope I helped, thanks for viewing my thread