Indicated, calibrated and equivalent airspeed are irrelevant here, we can convert directly from mach number to true airspeed.
The standard formula for the speed of sound is SQRT (Gamma x R x T)
Gamma for a diatomic gas such as air pretty much is, is 1.4
R, specific (something or other) for air, is normally quoted at 287 J/kg.K [sorry, I'm doing this from memory, I can remember the values but not the full name of the constant.]
T is in this instance fairly easy, it's the air temperature. Whilst layers vary a bit with conditions, you can reasonably assume that the aircraft(!) is in the lower stratosphere, which ISA quotes as a constant temperature layer at -54deg.C - 219.15K.
So SQRT (1.4 x 287 x 219) = 297 m/s
At Mach 15 then, this gives you 297 x 15 = 4,450 m/s
To convert m/s to knots: 1 m/s = 1.94kn.
So, Mach 15 in the lower stratosphere is 8,632 knots TAS (True Air Speed).
Well into the lower statosphere, you're well above most winds, which tend to exist primarily in the troposphere, and even the jetstream which sits around the tropopause (typically 30-50,000ft) so by and large TAS = G/S.
So, M=15 @ FL600 gives you about 8,632 kn (or 9,944 mph).
G
(Currently writing a chapter of a book on high altitude aerodynamics for a European Comission study group on high altitude UAVs).
N.B. I made the mistake of buying the audiobook of Angels and Demons for a long drive. Utter and total drivel I thought - is the film any better or worse?