A free app for Android, by Clean.

Ringtone Maker: Music Editor is a free application to create custom ringtones, alarms, and notifications from your favorite music. With Ringtone Maker, you can easily combine multiple audio files into a single ringtone or notification. It is also a very easy-to-use application, allowing you to trim, cut, combine, and remix your favorite songs and videos into a new song or new notification. It is completely free, with a simple and clean interface, and no hidden fees or in-app purchases.

You can trim and edit the audio file to make it shorter or longer, and you can add multiple audio files into one song.