I been using Dropbox as my primary storage solution for quite some time and I have never encountered this problem. There has to be a problem somewhere, because Dropbox doesn't convert your audio files into Word documents. Anyway, my advice is to download that file back to your computer. After the files have been downloaded, go to My Computer > (press Alt) > Tools > Folder Options > View and uncheck Hide Extension for known file types. Go back to where the file is saved and click once on it. Press F2 then input the real file extension of the file (MP3, Mp4, OGG, etc) and try playing it in a video player. If it does not work, then the uploaded file is corrupted.