The winds at the float altitude of 120k' (36km) have frequent and sustained gusts exceeding 25 m/s. The air is thin, and barely perceptible, but the effects of vortex shedding around structures is still vexing for those of us who try to keep payloads pointed and vibrations minimized. The PSD of the vibrations is sometimes significant in the lower frequencies. The PSD of the vibration profile shifts, sometimes dramatically, in real time to the measured winds and directional changes of the payload gondola at those altitudes.
As for a microphone freezing over, the environment at those altitudes will very quickly shed any moisture accumulated in any phase on exterior surfaces. A balloon typically rotates throughout its mission, and the extreme cycling between view factors for albedo, direct solar radiation, and deep space cycle any moisture accumulated in the tropopause or below in minutes unless the surfaces are somehow unusually shielded.