Mopidy frontends are for listening to core events and interacting with core. They don’t have access to the audio stream. You want to either use the audio/output setting to tee a copy of the audio or just see if you can read from the soundcard.
Either way you likely want this outside of mopidy if you need access to the audio stream to control the lights accordingly.
So, If I want audio/output, i need to create a backend extension.
I will see for read form the soundcard but I never do that so, I don’t know how do I do.
Maybe, if I have access to the uri/link with music and the time that the music are playing maybe I can create sync with my hue, with the api of http://the.echonest.com/.
No, @adamcik did not suggest a backend extension was what you are looking for. Mopidy provides no API to access the underlying audio data which is what you seemingly want to do. So you must access the audio data outside of Mopidy. The audio config section has an output setting which will allow you to configure something to intercept the data as it leaves Mopidy. The gstreamer tee element is a good place to start with this. You will probably find example configs where people have used a tee to output audio data to a Fifo which is then consumed by another program.