The problem as for right now is that the Web Audio API is not supported in mobile other than iOS 6+
Check this: http://caniuse.com/#feat=audio-api
This API is the base for building an audio visualizer.
If you are working with Phonegap you could make part of the application web and the visualizer part native and use ir as a Phonegap plugin. It is justified to do this depending how much % of the application the visualizer is.