Best answer by Airgetlam
Alexa doesn't know anything about your iPad, there's no way for it to connect to it and access music, since there's no audio input on the Dot.
That being said, all is not lost. You could start the playback using the Sonos controller from your iPad (Play from this device), and then control it using the Echo Dot for things such as pause, resume, volume, next, etc. In that case, your iPad is acting just like a NAS.