It seems they might have been listening in that location due to other data that they had available to them:
New satellite calculations have put the likely location of Malaysia Airlines Flight MH370 in the same spot where Chinese patrol vessel Haixun 01 detected deep water acoustic sounds on Friday and Saturday.
In the strongest lead to date, Air Chief Marshall Angus Houston, the head of the Joint Agency Coordination Centre, said the Haixun 01 picked up sounds coming from about 4,500m down, in two locations just two kilometres apart.
from
MH370: Chinese, Australian ships verify potential pings from missing jet