Is it compulsory to do an echo test if you know it's a Down syndrome baby?

No but recommended. Because many babies with down's syndrome also have cardiac or other anomalies, it is recommended to screen for those abnormalities in order to be prepared for the inherent problems at the time of delivery.
Yes. Echocardiogram, sonogram of heart, is routinely performed to make an early detection of heart abnormalities. More than one third of infants with down syndrome have heart defect (s).