Sound Classification for Multiple Classes for English Letters
If in each file the letters are spoken separately, with silence in between, and always in the same order (A,B...,Z) then one can try to automate finding each section and its label.
Use a Voice Activity Detection (VAD) module to detect each spoken character. Then assign A to the first voiced area, B to the next etc. If you have very clean and uniform audio, then it may work to use volume instead of voice activity detector.
Note that this process is a bit vulnerable to missed sections, because then all the subsequent labels will be shifted. It is also vulnerable to speakers saying characters out of order, or repeated.
An alternative would be to use a Speech Recognition system. That should be able to handle arbitrary order of utterances.