Some cultures use whistling languages to communicate when regular speech becomes ineffective over large distances. From Wikipedia:
Whistled languages are normally found in locations with difficult mountainous terrain, slow or difficult communication, low population density and/or scattered settlements, and other isolating features such as shepherding and cultivation of hillsides. The main advantage of whistling speech is that it allows the speaker to cover much larger distances (typically 1 - 2 km but up to 5 km) than ordinary speech, without the strain (and lesser range) of shouting. The long range of whistling is enhanced by the terrain found in areas where whistled languages are used.
Here's an mp3 of two men communicating via whistling. It sounds very much like R2-D2.
