Ring Studio Lite is the right application for you.
Just select the audio file, decide starting time, duration, put some effects and the ringtone will be done.
Among a wide range of media supported by this ringtone creator include: WAV, MP3, M4A, AIF, MP4, AAC, M4V, AU and CAF.
IMPORTANT: RINGTONE INSTALLATION NEEDS ITUNES SYNC
Ring Studio Lite works with any "DRM free" Apple supported codecs on any iPhone and iPod Touch.
Ring Studio Lite Features:
✓Select audio file from Ipod Library, internal Audio Recorder, Web Links
✓Ringtone's duration from 6 sec up to 30 seconds
✓Fade in and fadeout supporting
✓Customization of user interface by changing colors
✓Preview any ringtone with internal player
✓Mail created ringtones
What do I get more in paid version?
✓ No ads
✓Ringtone's duration from 0 sec up to 40 seconds
✓Change Tonality and Speed
✓Dropbox upload of created ringtones
✓Change background interface with up to 100 images