Go to Settings > Sound > Phone Ringtone/Notification and select your new sound.
Download the desired .ogg or .mp3 files from Zedge or a direct audio source.
You can listen to and download the full collection of notification ringtones from CyanogenMod 7 (Android 2.3.5). Iconic Sounds to Look For: CyanTone: The signature ringtone. Krypton: A popular notification sound. Pegasus: A classic alarm sound. Drip / Plastic Pipe: Classic short notification sounds. 🛠 How to Install Custom Sounds on Android