How to differentiate an empty aerosol from a full one using only sound measurements?
What I would do is :

1) Acquire the audio signal (or in fact, several, like ten) corresponding to a full container. I'd then quantify the energy of the signal, by squaring it and dividing by the number of samples. The obtained quantity is my reference energy. If several signals were measured, I take the average of calculated energies. Let's call it $E_{full}$

2) Do the same with an empty container (which is basically measuring ambient noise if it does not emit any sound like you say). Let's call it $E_{empty}$

From this point, you can define any energy threshold, with which you will compare the energy of the sound emitted by your experimental aerosol.

You can also do it simpler, by detecting for example the maximum peak amplitude of the full and empty aerosol and defining peak-amplitude threshold accordingly.

The threshold definitions can also be fine-tuned by measuring other aerosols with varying fillings, like half-empty, 25 % etc...

And finally, all the measurements I suggest implies that the sound is measured in steady state.