async fadeAll ({ state, dispatch }, { from, to, duration }) {
      const fades = Object.keys(state.tracks)
        .map(track => dispatch('fadeTrack', { track, from, to, duration }))

      const ids = await Promise.all(fades)
      return ids.flat()
    }
Fades all tracks between two volumes.

@param opts {object}
@param opts.from {number|undefined} start volume between 0 and 1
@param opts.to {number} finish volume between 0 and 1
@param opts.duration {number} duration of fade in milliseconds

@return Array of sound IDs that were faded