Optimise Sound Engine #1542
Description
Describe the Bug
Currently the sound engine is initialised when Webots is started, but in 99% of the cases this is not needed (sound is disabled by default).
This is therefore not efficient because it takes time and consume resources uselessly, furthermore sound cause many disturbing 'ALSA' warnings in Webots on computer without sound devices (on travis for example, but also reported by many users).
Instead, we should initialise the sound engine only the first time it is actually used.