A bug in iOS 8.1.1 has caused many people to lose their alert tones and ringtones. To restore these tones, first update your iOS device to 8.1.2, which was released yesterday. Then, do the following.
If you sync your device with iTunes, connect it to iTunes, and click on it in the iTunes navigation bar. Click on Tones in the sidebar, and make sure that Sync Tones is checked.
Click Sync to sync the tones to your device.
You can also do this from the iOS device, restoring the tones from the iTunes Store. To do this, go to itunes.com/restore-tones in Safari; this link will redirect you to the iTunes Store app. Tap Restore; You’ll see a message saying:
“This may take a while. You can continue to use this device while we prepare your tones. You will be sent a push notification when they are ready to download.”
Tap Done, and wait for the notification. When you see it, tap Download, and all of your tones will download. You can check that they’re all on your device by going to Settings > Sounds > Ringtone and Settings > Sounds > Text Tone (and the many other sounds listed).
See Apple’s support document about this issue.