Final answer:
Shelley uses the 's' consonant sound in 'Ozymandias' to emphasize the vastness of the desert, with the sound occurring in words that contribute to the sonnet's theme of desolation and emptiness.
Step-by-step explanation:
The consonant sound that Shelley emphasizes to convey the vastness of the desert in Ozymandias is the 's' sound. This sound, often spelled as <s>, can evoke a sense of whispering, stillness, or hissing, which contributes to the vast, empty feeling of a deserted landscape. The 's' sound repeats throughout the sonnet in words such as 'sand', 'stone', and 'stands', helping to convey the desolation of the ruins of Ozymandias's statue that once stood in the midst of a now-vanished empire.