If you are signed up for the Midwest Crane Count (Saturday, April 17th from 530am-730am) here are some resources to know what sounds you are looking for.
From the International Crane Foundation, a bio on Sandhill Cranes which includes sounds.
The Macauly Library of Sound has a ton of sounds and video but I couldn’t find any specifically with the unison call. Just search for Sandhill Crane and there are a few hundred results.
Please feel free to add in the comments any additional info on Sandhill Cranes you find and make sure to report on what you see! (Not just cranes!)