I grew up in (and still attend) the Old Regular Baptist church and can tell you Harvard University did a study on lined out hymnody (do a web search on that phrase for further info), with examples from a Scottish denomination that still practices it and the black churches of the deep south who also practice it.
The difference between the Old Regulars, the Separates, the Uniteds and the Primitives (as far as I can understand it) is the Primitives are more "pre-destinarian" in their views. The Baptists in Kentucky/Virginia used to be more or less one but split in the late 1800's over some doctrinal issues that sound like, to the outsider, more like splitting hairs than anything, but are central to their doctrines.
If you get a chance, read Dr. Howard Dorgan's EXCELLENT book "The Old Regular Baptists of Central Appalachia: Brothers and Sisters in Hope". Also, if you'd like to hear some samples, go to the website at www.oldregularbaptist.com.
Gary L. Eldridge