Some Song Research

 Ovenbird

Habitat (foliage density) influences song. Height is correlated with pitch, Cerulean Warblers are found high in trees and have a high-pitched song. Hooded Warblers and Ovenbirds are found low in the forest and their calls are louder and lower. Low-pitched pure tones travel better in dense vegetation. High, clear pitches do better in open areas; nightengales keep low in bushes where their buzzy song is less affected by wind and temperature.

Vocalization has motivation: aggressive = low and harsh; appeasement = high and soft. Why? Aggressive vocalization is replacement for combat. A bigger bird should tend to have a lower and deeper sound so you should advertise your bigness with that sort of call. On the other hand, baby birds sound high and attractive. High pitch = non-aggressive signal. Does this apply to human vocalizations?
Much research has focused on variability or organization or subunits of song: