Thank you for the details of the issue you are experiencing with audio playback when playing videos in iTunes. I found a couple of article with some steps you can take to address the issue you described.
I recommend reviewing the following articles:
Thank you for using Apple Support Communities.
Thank you for the additional information. Due to the nature of your issue or question you may find more information by using another one of Apple's support channels - https://getsupport.apple.com/GetproductgroupList.action
All the best,