Aircraft noise.

[Tom-D]

When a non jet aircraft flys over, are you hearing the prop or the engine?

 

[PeterNSteinmetz]

All acoustic energy (sound) travels forever but looses energy to the medium around it as it propagates.

There may be some other limits that will ultimately limit detectability. Turbulent mixing of the medium and ultimately, at low enough levels, the continuous approximation will break down, and then one is talking about potential quantum effects on single molecules.

(Rather far afield from the OP)