It would seem unlikely that one could secure copyright of the musical note A 440 (for example). Generally, more complex sounds (music, sound effects) are complex permutations, mutations or compositions of other, more basic sounds.
We also know that a sound is essentially a vibration of air. The sound exists at different states (when it is "played", when it is "composed", when it is "heard" and at other times and in other states). The same sound frequency will be slightly different in each of these states. For example, if you listen to a piano being played, the sound is different 1 inch from the Piano than it is 100 metres from the Piano.
The question then becomes how complex does the sound have to be for it to be copyrightable and what is it that you are securing? Are you securing the sound as it is played or as it is heard? How mutated or composed does a note (such as A 440) need to be before it is copyrightable?
Does the law take a scientific approach to trying to answer this question or has it been largely built on broad, casually accepted defintions of 'music'? i.e A lawyer thinking that something sounds enough like something else to be the same ...
Further does the law break sound down into lower constituent parts when attempting to answer this question? i.e What is the Frequency of the sound, the waveform of the air vibration, the pitch, the timbre, the sonic texture? etc. Is there a definitive list of lawfully mandated measurements which would be used to determine the 'uniqueness' or 'similarity' of a sound?
(Feel free to answer the question in relation to the law in the country in which you reside)