I would say that " yes, they'll " is incorrect, because of sentence stress.
2.
In spoken English, the homonymy disappears as sentence stress overrules the differences . talk ) 19 : 11, 24 February 2010 ( UTC)
3.
Since you're marking " will " as one of the stressed words using sentence stress, it is impossible to pronounce it " they'll " and therefore should be impossible to write it this way .-shoe 21 : 38, 1 September 2011 ( UTC)
4.
Her vocalizations were still soft and breathy, and they wrote that, " it is still very difficult to understand her if you have not been with her for a period of time . " In mid-1975 she could speak with a relatively normal declarative sentence stress pattern, and began to do so with increasing frequency.