Disable sound effects by default

[richelbilderbeek]

Is your feature request related to a problem? Please describe.

We have our first sound effects. Cool!

However, when we work on the game, that should be in silence.

Describe the solution you'd like

Like the music, the sound effects should be off by default. Only if --music is added as a runtime argument should we hear both music and sound.

If you think it'd be better to split sounds and music, add a --sounds as a flag to enable the sounds.

Describe alternatives you've considered

None.

Additional context

Add any other context or screenshots about the feature request here.

[rijkvp]

Travis is fixed but don't merge yet. I am working on something else.