iCoolsoft iPhone Ringtone Maker for Mac is an audio edition tool. It can be used to create ringtone files out of video or audio files. The application has good input format support, and you shouldn't have problems opening the most popular formats. It can convert those files to M4R, which is the format used by the iPhone for ringtones.
The first step of the process is to load the input file. You can do this by clicking on the "Browse" button and selecting your file. Then, you will notice there is an audio player in the middle of the screen. There are two triangle buttons below a timeline. You can use those to trim your input file to a shorter clip. Ideally, you would want your ringtone to be about 30 seconds long. There are three playback control buttons: play, stop and play selection. After you have decided what part of the input file you want to use for your ringtone, you can choose where you want to save the file. The app can automatically import your ringtones to your iPhone if it is connected, although I didn't test this feature.
On my testing machine, which runs Mac OS X Lion, I was unable to get this app to work. Every time I wanted to play audio or generate a ringtone, the application crashed. I tested the app on Snow Leopard and it worked just fine. I think this program hasn't been updated in a while. If you look on its website, there is a note that says that the app has been updated to support iOS 4.3 and iPhone 4. iOS 4 is probably 2 years old now, and Mountain Lion didn't even exist back then, and maybe that is why I couldn't get the app to work.