Yes, you can! The Smart Player is a great tool to use with audio-only files, since it provides the user with an interactive searchable transcript that plays along with the media, in addition to captions. You can submit Dropbox URLs to MP3 files, or other URLs pointing to MP3 files (e.g., http://www.domain.com/path/video_filename.mp3), as List of URLs Captioning requests. Once the request is complete, the audio will play in the Smart Player, and you can use the interactive transcript and the captions to follow along.
See an example of the CaptionSync Smart Player playing an audio-only file.
Learn more about the Smart Player feature in our tutorial.