I don't know
I got a ringtone maker app in itunes store called rmaker that you can pick 30 - 40 sec of the song. I can listen to a song useing itunes song purchased and songs imported and then tell it what part of the song to start at and how many seconds of the song I want it to use and save. Then all you have to do is sync your phone to the computer and then the ringtone you made will be on the phone. Good thing is it will give instruction all the way through and iTunes will help you put the ringtone on your phone. Takes few minutes.
I know ringtones can be a fun way to customize your iPhone sherill! You should be able to assign the songs you purchased if they were in ringtone format. Have you tried accessing the "settings" menu and then select the "sounds" option followed by the "ringtone" option. You should see the available ringtones there. I have attached an additional support link for your reference. http://bit.ly/9emtRK
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