It's really a non-issue in my opinion since there is so little storyline in the coop mode. But if you want to hear all of GLaDOS' comments (in coop mode), you can watch these videos: 

[youtube.com/watch?v=IwYSxnV0ycU][1]  
[youtube.com/watch?v=R4bE9uyru4k][2]  
[youtube.com/watch?v=rINE2tQrcvg][3] 


  [1]: http://youtube.com/watch?v=IwYSxnV0ycU
  [2]: http://youtube.com/watch?v=R4bE9uyru4k
  [3]: http://youtube.com/watch?v=rINE2tQrcvg