Just a quickie, Rick:
It's Instrumental/Ablative: singing by with or from the heart. Heart is
singular because it's one man, one heart. And it's "the heart" not "your
heart" because with parts of the body, it can only be yours that you control!
In Col 3:16 it's plural, because the writer forgot the "one man, one heart"
convention. Koine Greek often uses a redundant preposition instead of the bare
oblique case, ISTM.
HTH.
-- Revd Ben Crick, BA CF <ben.crick@argonet.co.uk> 232 Canterbury Road, Birchington, Kent, CT7 9TD (UK)