Custom Ringtones, Alarms, Notification tones in Android

To add custom sounds to your Android phone, create the following folders on your SD-card:

/sdcard/media/audio/ringtones
/sdcard/media/audio/alarms
/sdcard/media/audio/notifications

Now you can put your MP3 sound files into these folders and they will appear upon the tone selection dialogs. (You may have to wait a while until they appear, maybe even have to reboot the phone.)

On older Android phones, you might have to create the folders ringtones, alarms and notifications in the root of your SD card. (see here)