Is listening to a Dharma talk while walking considered a form of walking meditation?
Yes, in walking meditation we can use any of the 40 objects of meditation. 

Listening to Dhamma talks would put it under recollection of the dhamma  as it gave you a sense of appreciation of the truth of the dhamma, IMHO,rather than the investigation of Dhamma (Dhammavicaya).


  "Mahanama, you should develop this recollection of the Dhamma while you are walking, while you are standing, while you are sitting, while you are lying down, while you are busy at work, while you are resting in your home crowded with children.