The idiom in the sentence conveys the General's anger at being tricked.
The role of the idiom in the sentence from Mark Twain's 'I Sell a Dog' is to communicate the General's anger at being duped by Twain. The idiom 'for a moment he couldn't seem to find his voice' is used to show the General's surprise and speechless reaction upon realizing he was deceived by Twain. It serves to highlight the General's emotional response in the dialogue.
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