Devil's Advocate: Not being able to talk to those outside of the dungeon keeps you focused on the task at hand. It keeps players focused and moving forward through the dungeon rather than trying to chat/fight/run at the same time. In addition, the sound notifications some players may have could confuse them thinking the sound was a trigger caused by a boss attack or something in game.

My solution: Make it optional to not be bothered while you're in a duty. Those who start falling behind from not paying attention can be vote kicked. Problem with this, like anything, it can be abused. It's reliant on player maturity and discretion to use the tool responsibly.