What is the r-value of 2 foam sound board?

I assume you mean 2 inch thick foam sound board??? The answer lies in what type of foam specifically you are speaking of.Polyisocyanurate foam has an rvalue of approx. r-7 per inch,being one of the higher r value foams readily available in 4'x8' sheet size.Polystyrene foam board,commonly found in building centers and home centers,and commonly referred to as "Dow" board has an r value of approx. r-5 per inch.To calculate the 2" thickness you reference,simply double the r value given(r value is generally stated as "r value per inch".Thus ,2" polystyrene has an r value of r-10 (r-5/inch x 2).These are fairly general guidelines,and depending on your application,you may want to find more detailed specifications that speak to degradation of R value over time,properties and characteristics of various foam types,etc,etc.