If you don't want to download content of the file then waveform image or data must be prepared on server.
Some time ago BBC (yes, UK media company) open-sourced set of tools to do it efficiently. All is described on their blog: http://www.bbc.co.uk/rd/blog/2013/10/audio-waveforms
Shortly: on Linux server-side you have to convert sound file (FLAC, WAV or MP3) to JSON-based waveform interpolation with audiowaveform command-line tool. Next you serve the JSON data to browser client which will render waveform on canvas element with waveform-data.js.
