Does thinking out loud negatively affect one's ability to think internally?
Yes, anti-correlation between extrinsic and intrinsic brain networks may be altered by different practice. I give an example of how meditation affects anti-correlation between extrinsic and intrinsic brain networks.


  "Human experiences can be broadly divided into those that are external
  and related to interaction with the environment, and experiences that
  are internal and self-related."
  
  "Anti-correlation between extrinsic vs. intrinsic systems was stronger
  during FA (focused attention) meditation and weaker during NDA (non
  dual awareness) meditation in comparison to fixation (without
  mediation). However, correlation between areas within each system did
  not change across conditions. These results suggest that the
  anti-correlation found between extrinsic and intrinsic systems is not
  an immutable property of brain organization and that practicing
  different forms of meditation can modulate this gross functional
  organization in profoundly different ways."


Given these findings, your practice of speaking out loud may favor extrinsic brain network paths which could explain your loss of clarity when you suppress "thinking out loud" while among other people.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3250078/