1. A method, comprising: at a computing device having one or more processors and memory: receiving a text input from a user, including receiving copied or scanned text for which context-appropriate phonetic annotation is to be performed at the computing device; identifying a first polyphonic word segment and a first monophonic word segment in the text input, the first polyphonic word segment having at least a first pronunciation and a second pronunciation that is distinct from the first pronunciation, and the first monophonic word segment having a single pronunciation; determining at least a first probability corresponding to the first pronunciation being a correct pronunciation for the first polyphonic word segment and a second probability corresponding to the second pronunciation being the correct pronunciation for the first polyphonic word segment, wherein the first probability is greater than the second probability; determining a predetermined threshold difference based on: (1) a comparison of the first probability and the second probability with a preset threshold probability value, respectively, and (2) a magnitude of a difference between the first probability and the second probability; comparing the difference between the first probability and the second probability with the predetermined threshold difference; and selecting the first pronunciation as a current pronunciation for the first polyphonic word segment in accordance with a determination that the difference between the first probability and the second probability exceeds the predetermined threshold difference; and in a text presentation user interface, displaying the input text concurrently with context-appropriate pronunciation annotations to facilitate a user's reading the input text aloud, including: phonetically annotating the first monophonic word segment in the displayed input text with the single pronunciation of the first monophonic word segment; phonetically annotating the first polyphonic word segment in the displayed input text with the first pronunciation of the first polyphonic word segment; and forgoing phonetically annotating the first polyphonic word segment in the displayed input text with the second pronunciation of the first polyphonic word segment.