Revert "Adds stop all sounds playing"

[ffont]

Reverts #45 as it generates Uncaught ReferenceError: PropTypes is not defined error at load time. We should have automatic tests enabled (which I guess would have caught that).

[noVaSon]

Ah this is strange.

The original PR deletes the PropTypes import, this is where this reference error came from.
Just add import { PropTypes } from 'prop-types'; and it should work again.