Virtualbox `audio` can have different values depending on OS

Bug is [here](https://github.com/tinyfm/sandbox/blob/master/Vagrantfile#L42).

Issue referenced on [modipy vagrant](https://github.com/paulsturgess/mopidy-vagrant/issues/3):

>  `coreaudio` is only available on a Mac OS host. 
> For a Windows host use `dsound`. 
> Linux gives you a couple of options (`alsa`, `pulse`).
