Hi Jack,

A possible correction to what you've heard: The rabab made reference to in the Sikh tradition is a different instrument. It is essentially an obsolete instrument, save for the fact that nowadays many musicians in Sikh sacred music are reviving it, perhaps. Here's a photo I found of what looks to be a replica of that old style:
Rabab of Mardana

The other thing to note is that the rabab is remembered as the instrument of "Mardana" who was Guru Nanak's minstrel. Mardana was a Muslim by birth that came from one of the hereditary ethnic communities of bards/musicians. Since Nanak devised all his poetry (later, the scripture) to be sung, as was the custom of the time, it was also customary that a tradition bard be the voice of that. Interestingly, most of the drawings/paintings depicting Mardana playing the rabab make it appear as if he was maybe only using it as a drone instrument! However, painters have been known to be ignorant of instrument playing postures, too :)

Gibb