There have been many attempts to classify sounds in the same way that one may classify, for example, birds. Pierre Schaeffer's Traité des Objets Musicaux states that a sonic object is not the instrument that was played, nor the medium itself (in his day, magnetic tape), nor a state of mind. This project is surprisingly difficult, and will waken up the ears to previously unconsidered aspects of the sounds.
Make or download a collection of up to six sounds. Find sounds that are as different as possible from one another. Listen to them repeatedly under studio conditions or on headphones. Try to hear every last detail of the sound, then: Classify the sounds according to what they are Classify the sounds according to what they are not.
Of course, sound is not really divided into 'objects' but is rather a continuum. One question to ask during this project is to what extent the recording or sampling of these sounds removes this sense of continuum. Also, ask whether the concept of a 'sonic object' is useful.