First get your sound in an .mp3 format.

Then open iTunes. Choose File (Alt + F) > Add file to library > Browse for your file and then click "Open."

Next go to your music library.

Choose "Library"  in the upper right if in the iTunes Store > Choose "Music" in the upper left > Choose "Songs" in the upper middle.

Sort by Date Added most recent on top. If you don't see it there then right click on the column headers and a list of options will appear for sorting. Choose Date Added.

Once you've sorted by date added you new file will be on the top.

Right click the file and Choose "Create AAC version"

The new AAC file should appear in your library. Right Click "Get Info" it will tell you the location of the file.

Next browse on your computer to the file.

You need your folder options to unchecked for "Hide known extensions"

Then replace the .m4a to .mr4

Go back into iTunes do the add file to library thing again and whala. It's a ring tone in your iTunes.