Maybe Ravan's stringing Veena with his sinews is really accurateI brought a guitar few months ago. Guitar is a lute and lutes usually have six strings. 2-3 bass strings for low voices and rest are thinner strings for high voices. Modern strings of any stringed instruments are either made of steel or nylon. But back in medieval days and even some ancient lutes, animal guts were used as strings. To make the voices low, guts were stretched and lengthened. This would make low voice playing incredibly difficult as the neck of the instrument gets really long. No one had a good solution for it until in 1700s we got wound steel strings. These strings were wound around a hairy structure and produce lower sounds staying the same length as other strings. This was revolutionary and today all stringed instruments today use wound steel strings for bass notes. Nylon strings came in 20th century and occupied the same place.
But Veenas are believed to be as old as 2000 years ago. They are mentioned in vedas. So apparently like other lutes of that time Veena was stringed with animal or plant fiber. But since plant fiber can damage easier, there is a high possibility that Veena was strung using dried animal gut(most probably from goats and sheep). So Ravana stringing a Veena with his own sinews is pretty accurate.