Depends on the device.
Nexus devices and Samsung utilize ogg files for notifications and store them within the root files. Specifically, System -> Media -> Audio -> then notifications or alarms or ui or ringtones folders for specific sounds. The System -> Media folder is where you will find the boot animation file, keyboard tapping sounds, boot up sound and so on.
You can also install Google Wallet on any device, once you root and gain access to this area of your file system.
P.S. Don't forget you need to alter the owner and permissions of the sound files to match what you replace.