async muteAll ({ state, dispatch, commit }) {
      const mutes = Object.keys(state.tracks)
        .map(track => dispatch('muteTrack', track))

      commit('setMute', { track: 'all', muted: true })

      const ids = await Promise.all(mutes)
      return ids.flat()
    }
Mutes all sounds regardless of current mute state.  Maintains volume
settings on sounds.

@return IDs of sounds that were muted