So, to summarize / paraphrase / highlight after bunch of listening,

Pilot contacted ATC (Departure) right after takeoff asking to level off at 8000'

ATC granted that then asked a few times what was going on and their intentions. There was a long length of silence and repeated inquires by ATC to contact them.

Responses were garbled and generally inaudible to ATC.

ATC eventually pulled out of them that they wanted to return and they had "smoke and fumes and were using masks", masks were cause of the garbled transmissions. ATC gave them return vectors and a 3000' altitude. It was never said whether "smoke & fumes and masks" were in the cockpit, cabin or both. 5 hours of fuel and 167 souls on board.

Several times ATC asked if they were declaring an emergency but I never heard a response. They were transferred to approach control just fine and continued to get many change of vectors.

They were asked to land on any runway and chose 31L. Their transmissions became very clear at this time and I assume they were off mask.

At least twice ATC asked if they wanted to begin a turn towards the airport. They declined, stated they needed a few more minutes. ATC notified them that the were traveling going beyond the class B airspace but it was OK and to let then know when able to turn towards 31L.

They eventually asked and were cleared to turn towards 31L and asked if the emergency trucks could be cancelled as the situation was under control. They were in place already. They were switched to tower.

In the meanwhile much traffic continued to land, take off and taxi on 31L and 28, intersecting runways.

The trucks asked for permission to trail the plane on the runway once landed and were given permission to do so.

The plane got off the runway quickly, by the deicing pads and were transferred to ground control. I was unable to follow after that point. Flight Aware implies they touched down at 10:14p and were at the gate at 10:44p

To quote an FT Evangelist, let the speculation begin..