I saw that an earlier thread on this topic had been closed. I just wanted to let anyone wondering about it and the writers of Rick's Italy guidebooks know for certain that the monks no longer chant at Sant'Antimo. We were there earlier this September and there were just recorded chants playing. We were told that the monks who used to do the chanting returned to their home abbey in France in 2015. It's still a beautiful place and well worth a visit. We were fortunate to be there in 2014 and hear the monks. It is one of my favourite travel memories.
The best way to let the guidebook guys and gals know of this - because they aren't conspicuous here and may miss this - is to complete the guidebook feedback form at https://www.ricksteves.com/forms/guidebook-feedback-form
Thanks! Will do.