For some reason, when you talk about using a Cello in battle it reminds me of a Woody Allen movie where he talked about playing either the Bass or Cello in a marching band. It showed him scurrying forward with instrument and chair in hand, setting the chair down and playing a few notes, then getting up and running again as the band marched on around him.
Back on topic though, the bow of a violin or cello is strung with horse hair (from the tail, for length). Why? Because hair has an odd property: It has scales on the surface, and they all lay the same way.
Yeah, if you take a long hair you'll find you can pinch/slide your fingers in one direction pretty easily, but not in the other. It's this property that makes the hairs vibrate the strings as it slides across.
So you can't use razor wire to string the bow, not if you want it to make any sound. (Unless, of course, you happen to be in a fantasy game. Then anything goes.
Even if you could, every time you swing your bow/blade/wire thingy, you've stopped playing the instrument.
Consider using Perform: Voice for your bardic music checks.
Or, if you want to be really badass, take Perform: Drum. Bard songs can affect anyone within hearing range. Drums are used to communicate across miles of distance. So let your Bard use Inspire Courage to affect an entire military-scale battlefield all at once. Give each and every soldier in a Legion (5,000 men) a +1 or +2 bonus to hit and damage and you've definitely earned your pay for the week.