That's right, the speed of sound is given by rt(gamma * R<a> * T)
Where gamma is the ratio of the specific heats, R<a> is the gas constant for air, and T is temperature absolute. Bear in mind that the unit for length here depends on that used in R.
Under SI:
Gamma for air is 1.4, R for air is 287.05 m^2/(s^2 K) and ISA T 288.15 Kelvin.
Under Imperial:
R for air would be 3,089.8136 ft^2/(second^2 K)
If you were to use a value of 1084.6391 for R<air> then the dimentions would be nm^2/(hour^2 K) and thus the above would yield a speed in knots.
All of this nicely assumes that we are dealing with an ideal gas, which for the normal pilot's application is fine. One last point while I'm beating the issue to death, the speed of sound at sea level under ISA is referred to by the Greek letter alpha with a subscript 0. a<0> * rt(theta) yields, as stated, a local speed of sound. theta being T/T<0>. Again, this subscript 0 indicates the ISA MSL value (288.15 Kelvin.)
A wonderful article may be found here,
http://www.jeminas.com/aviation/pdf/...ht_Testing.pdf and section 4.3 would be relevant.