Don't play sound when muting

Summary: When the user presses the mute key to mute the sound they explicitely don't want sound. E.g. when they are in a public place and mute their system playing a sound is awkward

Test Plan:
On an unmuted system press mute button -> no sound played
Press mute button again -> sound is played

Reviewers: drosca, #plasma

Reviewed By: drosca

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21045