Sound emitted by droplets hitting a liquid surface when a liquid jet (e.g. water) hits a water surface sound may be produced if, due to Plateau Rayleigh instability, the water jet has formed into droplets.
But what exactly causes the sound? Is it the trapped air bubbles popping when they rise up to the surface again? Does the size of the bubbles matetr? Or is it the drops actually hitting the water surface breaking the surface tension? Or both? Please help me out on this one.
Best Regards from Berlin
 A: Air bubbles are the primary source of sound in pouring water, but the form of the surface and the container also contribute to the properties of the sound. In the freely available paper by Zheng and James 2004 entitled Harmonic Fluids, the authors state:

Ironically, the complex visible motion of the air-fluid interface
  causes relatively little sound, in part because visible surface
  motions are inefficient radiators of sound waves at audible
  frequencies [Bragg 1920]. Instead, the fluid shape vibrates
  harmonically at audio frequencies due to the microscopic oscillations
  induced by internal air bubbles, and acts like a shape-changing 3D
  loudspeaker.

The model, itself (which you can actually listen to examples of here) relies on a dual-domain Helmholtz Green's function, such that it includes two boundaries:


*

*the boundary of the liquid and it's container, Γs

*the boundary of the liquid and the air (where our ears are), Γa.



Zheng and James, Figure 8
This is how the form of the container affects the resulting sound. However, as I understand it, the size and number of bubbles has much more influence over the source strength and frequency of the pouring event.
