The acoustic signal is absolutely central to the study of speech. We can use many different ways to produce the sounds of speech (including using machines), but the signal itself is what carries the message to the listener.
When we speak to each other the sounds we make have to
travel from the mouth of the speaker to the ear of the listener. This is
true whether we are speaking face to face, or by telephone over thousands
of miles. What is important for us in our study of speech is that this
signal is completely observable: we can capture in the form of a recording
everything that the listener hears, and then measure whichever aspect of
the signal that we want to know about.
Because the study of speech acoustics is often presented in terms of some quite difficult maths and physics, some students feel afraid of the subject, but the basic principles are quite simple and quite easy to relate to traditional phonetics.
Something that needs to be remembered is that almost all books and papers in the development of the study of speech acoustics have taken the adult male voice to be the "standard" or "normal" voice. Female voices and children's voices have been much less fully studied. This regrettable fact shows a major male bias in this subject, and it will take a long time to correct. It should not need pointing out that adult males form a minority within the community of human speakers.