async unmuteAll ({ state, dispatch, commit }) {
      const unmutes = Object.keys(state.tracks)
        .map(track => dispatch('unmuteTrack', track))

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

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

@return IDs of sounds that were unmuted