In the folk field, as well as in the less sure-footed literary Lallans, Scots may be said to 'include English, and go beyond it'.
Hamish Henderson. The Ballads in 'A Companion to Scottish Culture', 1981.

Depending on the kind of song being sung, the language of Scots folksong can differ quite considerably.  The classic narrative 'Child' ballads, or muckle sangs, tend to be sung in a less regionally-distinct 'ballad Scots', whereas other types of songs, e.g. bothy ballads, can be quite regionally specific in their dialect. There are no hard and fast rules however, and various factors come into play in determining the level of Scots used, such as the influence of printed versions or the intended audience. Many songs in Scots-speaking areas have a strong print tradition, having moved around with the growth of cheap presses, and as such are often found in standard English, alongisde other songs in the local dialect. 

Further reading:
Hamish Henderson, 'At the Foot o Yon Excellin Brae: The Language of Scots Folksong', in Scotland and the Lowland Tongue, ed. J. Derrick McClure, Aberdeen University Press, 1983.