First, follow the steps of the original method
- Create your ringtone, save it as an AAC file.
- Rename the m4a extension to m4r.
- Double-click to play in iTunes.
- Go back and re-rename that m4r file back to m4a. That's it.
- Plug in your iPhone and that ringtone will be added to your syncable ringtones list -- and it won't pop up the error from before.
- Play your ringtones constantly for the next 10 hours until 7.4.2 comes out.