The velocity of sound in air depends on temperature as v∝T (absolute temperature).
If the velocity doubles:
v1v2=2=T1T2.
Squaring: T1T2=4.
With T1=0°C=273.15 K and T2=273.15+α K:
273.15273.15+α=4, giving 273.15+α=1092.6 K.
Therefore α=819.45°C≈819°C.