Feature extraction in audio spectrogram
The "ch" sound is called a phoneme. Automatic speech segmentation on the phoneme level (as opposed to word or sentence) will allow you to extract each phoneme from the speech samples. This is sometimes referred to as phoneme segmentation. Several papers for this are available in the literature.

Once you have the phonemes extracted, you can compute distances/similarities between them (in some feature space). An example feature representation could be MFCC.