I think Carbon Fibre is expensive - just check F1 race cars, and others that use it....
Comparison of carbon fiber vs steel manufacturing costs
The advantage is strength for weight - Carbon is Light and durable and won't corrode. But any damage at all to the Carbon Fibre - just chuck it away - not repairable as far as I know.
Carbon Fibre has some reduced vibration characteristics as well, so I suppose it will focus the sound very well. But cost would be high.