The music to which traditional ballads were sung has, for a long time, been mostly ignored. Ballad lyrics were set down in a number of manuscripts, but not many people bothered to actually set down the tunes to which the ballads were sung. During the "folklore renaissance" which occured at the end of the last century, during which scores of ballads were collected and set down for fear of their being lost entirely, music wasn't considered as important as the "literature" of the ballad lyrics.
It is rare even today to find a book that contains both a good selection of traditional border ballads and a good selection of music. This lack is partially a result of the lack of surviving ballad tunes, and partially because of the traditional emphasis on lyrics. But whatever the reason, it's frustrating.
The ballad tunes found in modern books were taken down, for the most part, in the 19th century. Some few were discovered in 18th century manuscripts, and a precious few tunes can be documented in 17th century sources. Farther back than that, however, one draws a blank.
However, music was an integral part of the ballad. Often times new words were set to traditional tunes, in a sort of period "filking". As the metrical structure of ballads was so similar, this would be easy to do. But reading a ballad and singing it to a tune have two completely different sounds.
Take the ballad Lamkin, for example. It has a very even sound when spoken: "Lamkin was a mason good as ever built wi' stane.". But when you sing it to the melody, which is in triple time, the rhythm and emphasis of the words are very different. The verse of four lines that the ballad is traditionally written down as reveals itself as not four, but two musical phrases. The high note at the end of the first phrase draws us into the second.
When setting your ballad to a tune, you have two choices. You can set it to a tune used for a different ballad. This is technically the more "period" choice, although all extant ballad tunes were written down long after the ballads themselves were created. Or, using the ballad melodies you can find as a template, you can write your own tune.
Almost all Ballad music was created in one of three scales. In order of frequency, they were: Ionian,Dorian, and Aolian. For the non-music majors out there, Ionian is your basic C major scale. Dorian is a type of D minor scale--if you have a piano, start on a D key and play every white note till you get to the D an octave above. (Lamkin uses a dorian scale). Aolian is a close relative of A minor--start on the A key of a piano and play every white key till you get to the A an octave above. Aolian and Dorian modes, because of their resemblance to minor keys, have a sadder and more melancholy sound; Ionian melodies sound more cheerful.
In addition, many ballads were pentatonic, consisting of only five notes plus the odd incidental (a note sharpened or flattened by a half-note). Lamkin, as well as being in the Dorian mode, is a pentatonic melody: it only uses the notes D,E,G,A,and C.