I'm only part of the way through the act at the moment but of the people we can talk to we have the following groups:
The tieflings: they are all fleeing from Elturel following the events of the Descent into Avernus adventure module. Since they are from the same place it makes sense that they have similar accents to each other
The druids: a group of individuals presumably local to the area
The goblins: another group that is presumably local to the area
The party members: Gale is from Waterdeep, Lae'zel is githyanki (although would have grown up on the material plane), the rest appear to be from the Baldur's Gate area.
From interactions with the tieflings we can assume that the area in act 1 is somewhere along the 200-250 mile stretch of the Chionthar river between Elturel and Baldur's Gate. Accents generally vary by region and language. Common is a trade language, with Chondathan being the language of the region. I would therefore expect most people from the area (which is in the middle of nowhere) to have similar accents belonging to one group of accents, i.e. all British or all American accents. I would not expect to hear a wide variety of accents without good reason (and I don't think that "not human" is in general a good reason). I therefore wouldn't be surprised if we hear a greater range of accents when in Baldur's Gate, which is a major trading centre along the Sword Coast.
All fair points, except you have a multitude of different races. Which are dramatically more diverse than different ethnicities of Humans. That is why it makes no sense for Goblins, Drow, Hobgoblins, Tieflings, etc. to all sound the same. Drow exist miles beneath the surface, in what might as well be the other side of the planet. Moreover, Lae'zel did not grow up on the material plane, based on her available dialogue. Your arguments are fantastic for why they all would speak the same Language (Common) but not why they would all share the same accent.
We have a multitude of examples of varied accents even among similar locales in D&D, namely BG 1 and BG 2. It makes no sense why that also isn't the same for BG 3.
