The Gaelic practice of lining out Psalms migrated with the Scots to the Appalachian mountains. It was used by most churches in the area with the Baptists and some smaller churches continuing the practice. Old time music is a blend of lining-out vocals often in falsetto, Irish jigs, Italian folk traditions (which is probably why we see a lot of mandolin) and of course African rhythms and syncopation.
I have posted a few examples of Old Regular Baptists singing lined-out hymns on YouTube:
I took the photos while walking through the local cemetery, one of my favourite places to visit:
You can hear the influence of lining in the vocals of Holcomb.
Yours in the Lord,