This skill is unofficial and does not apply to the Dropbox directly. You need to link your Dropbox account to allow Echo access to audio files. .mp3, aac, wav, ogg, .ts, .m4a formats are supported.
AudioPlayer commands: Alexa, ask cloud player to play {query} files Alexa, ask cloud player to play all Alexa, stop Alexa, resume Alexa, next Alexa, previous Alexa, loop on Alexa, shuffle off