async fadeAndStopAll ({ dispatch, state }, opts) {
      const fades = Object.keys(state.tracks)
        .map(track => dispatch('fadeAndStopTrack', { ...opts, track }))

      const ids = await Promise.all(fades)
      return ids.flat()
    }
Fades all sounds and stops those sounds after fading.  Takes the same
options as fadeAll and stopAll

@return Array of sound IDs that were faded and stopped