If you already bought the full version, you don‘t get billed a second time. Just make sure you use the same AppleID.
Regarding the mp3: I have seen a couple of mp3 files with unusual encoder settings that fail to import with this error. Usually those files also can not be played in QuickTime. Some Kondor incompatibilities with Apples mp3 decoding. I would recommend you transcode the mp3 to a standard mp3 format like 256kbit fixed rate. That usually takes care of this issue.