Full revamp of the background soundtracks, including music for special rounds (like zombie swarm, hard mode, nemesis, etc.), last human, round start, and even the regular tracks that play during normal gameplay. I'll give a few examples;

Round start - Zombie Scenario - Rush, I previously created and edited audio files, trimming and converting them to ensure they seamlessly replaced the original server audio files. Specifically, I used this soundtrack for start1.mp3 and start2.mp3, sadly I no longer have them as I reworked the audio files using the soundtracks used in Shadows of Evil(BO3 Zombies). I also found it weird how some files were mp3 and some wav.
Nemesis - Zombie Darkness - Good Night, same with this one.
Zombie Swarm - Zombie Z - Round Start