mixer-control: Use the "emblem-system-symbolic" icon

As suggested by the last round of design review [1], use the
"emblem-system-symbolic" icon with system sounds.

[1] gnome-control-center#548
1 job for master in 4 minutes and 17 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed build-fedora #327051

00:04:17