If you want some custom notification sounds for your Nexus 6 "Justen7723" has you covered. There are sound mods that bring the sounds from Star Wars, Star Wars Vadar Edition, Star Trek, HALO, Cartoon, Portal, Assassins Creed, Transformers, Ironman, Mass Effect, Resident Evil, Dead Space, Super Mario Bros, Zelda, Metroid, Tron, Wolverine, Pacman, and many stock sounds from other devices.
There are a few different ways to install these sound mods to your device. If you are rooted, and have a custom recovery installed you can just download the file and flash it in recovery. The manual method still requires you to have root. You will then use a root file explorer to edit the system/media/audio/ui file. Head to the link below to download the files.
via XDA