I've only heard Mennonites sing in person on two occasions, and both times, it was unforgettable. They are wrong in insisting that God should not be worshiped with musical instruments, but their error has had a happy result. They have become near expert in singing in harmony. The only word that I know to describe it is "stunning".
For many reasons I could never be a Mennonite theologically, but I must say that you have never heard singing until you have heard Mennonites sing. But the true sound of their singing seems to be difficult to capture adequately in a recording (perhaps because it is so multi-dimensional, like the difference between hearing an orchestra in person and on CD). The recordings uploaded by S. E. Samonte have come close to recapturing the sounds I remember. Here's Rock of Ages for your worship.