$v = \sqrt{\gamma RT/M}$, so $v \propto \sqrt{T}$ (absolute temperature). At 0 degrees C: $v \approx 331$ m/s. Approximate formula: $v_t \approx 331 + 0.6t$ m/s (where $t$ is in degrees C, valid near room temperature). When temperature increases by $\Delta T$: $\Delta v/v \approx \Delta T/(2T)$. Humidity increases speed because water vapor ($M = 18$) replaces heavier N$_2$ ($M = 28$) or O$_2$ ($M = 32$).