Sound safety

Issue generated from Tuleap's migration script. Originally submitted by: (daoumilseitek)

A maximum sound volume must be set. Otherwise it is like giving your penpal control on your volume via mic volume, and an accident can be quite dangerous for your ears. I had that happen when using another messaging system, trust me it's unpleasant.

Now there must be some thinking about that, because of the many volume controls we have : system volume, app volume, headphone volume...