Numerical simulations of temporally-developing round jets at a diameter-based Reynolds number of 12,500 are carried out with the aim of investigating temperature effects on the formation of the steepened acoustic waves usually associated with crackle noise. One isothermal and four hot jets at a static temperature equal to 2 or 4 times that of the ambient medium are considered. The isothermal jet has a Mach number of 2 while the hot jets have either the same Mach number or the same jet speed as the isothermal jet. At a constant Mach number, the pressure levels and skewness factors are higher at a higher temperature, indicating a more pronounced steepened aspect of the acoustic waves. This increase is due to the rise of the jet speed from 2 to 4 times the ambient sound speed in that case. When the jet velocity is constant, lower pressure levels are obtained at a higher temperature, and a slight reduction of the skewness factor is observed. This reduction is explained by the decrease of the ratio between the convection speed and the jet velocity with temperature. The present study thus allows us to isolate the effects of temperature on the generation of steepened waves near high-speed free shear flows.