One of the key points about using WiFi speakers is you need a good signal to enable loss less music
Otherwise the system shall behave erratically and drop, stop, jump, crackle etc...
At each speaker if you have a WiFi signal strength app or Wifi sniffer, the signal strength should be less than 70dB which is good.
Its worth checking for any large metal pillars or obstructions which could cause the interference.
NOTE: if you are using your device to stream to the speaker directly, putting device in the pocket or in a cupboard can cause the music to buffer, crackle or stop as the signal between your phone and router is not sufficient.
Try putting the device into an open space and try again
Other articles about iOS device to try also : http://support.litheaudio.com/en/support/solutions/articles/44002027365-airplay-airplay-2-dropping-connection-stops-or-cracklesto