Around 1600 the long /i:/ sound of Guy, Guido (and of buy, blind, mice, etc.) was in the middle of changing to the modern /ai/. Authentic performances of Shakespeare, and of songs from the same period, use the diphthong /@i/ (where @ = schwa).

The effect is something like the French oeil.

A similar vowel is still heard in the West Country of England, where people will call you moey loev at first meeting.