If you have the issue with files, that you were previously able to play, try to reboot your phone. If playback works afterwards, you seem to have the same issue as described in this problem: Unsupported File Type: Why did my Samsung Galaxy S suddenly stop supporting MP3s?Unsupported File Type: Why did my Samsung Galaxy S suddenly stop supporting MP3s?
I do not know of a fix for this yet, the only workaround seems to be a reboot.