The compositions of the Guru Granth Sahib were sung from the beginning, and the carrying of that singing has its own history, distinct from the history of the text.
Hereditary families of musicians served the Gurus, and two of them composed the var in the Ramkali section. Their descendants and successors carried the performance tradition for centuries, and the transmission was oral: the measures, the melodies and the manner of setting a composition passed from teacher to student without notation, because there was no notation to pass.
That transmission was disrupted repeatedly, by persecution, by the collapse and reconstitution of institutions, and severely by the partition of the Punjab in 1947, which cut communities and lineages apart along a new border. What survives today reaches back through those breaks rather than around them.
The consequence for anyone asking how the Gurus' compositions originally sounded is that the question is not answerable with confidence. Sustained efforts through the twentieth century and since have worked to recover and standardise the performance of the rags as the Granth specifies them, and different lineages hold different readings of the same measure.
The text is not affected by any of this. The headings, the order and the pagination are fixed and have been for centuries. What was never written down was the sound, and that is the part that has to be held by people rather than by paper.