Final answer:
Pronunciation is the linguistic feature not identified as affecting a speaker's credibility, whereas rate of speech, volume, and pitch are recognized as influential.
Step-by-step explanation:
The linguistic feature not identified in the textbook as affecting a speaker's credibility is pronunciation. While aspects such as rate of speech, volume, and pitch are recognized to impact how a speech is perceived and therefore can affect the speaker's credibility, pronunciation was not mentioned as having this effect.
Rate of speech is related to the speed at which one speaks, volume refers to the loudness of the speech, and pitch relates to how high or low the speaker's voice sounds. Proper control and utilization of these elements can enhance a speaker's persuasiveness, expressiveness, and ultimately, their credibility.