Share your experience!
Try to play one of the files in walkman,see if the phone manages to play them.
Try to force turn off the phone:
Remove the SD card and put it back in.Turn on the phone.
How are you trying to change your ringtone?Are you changing from walkman directly or from settings-sound-ringtone-you have a little music note in right corner,tap on it and select from your own music.
I am glad you sorted it out.Anytime.