Why does Obi-Wan’s accent sound so strange in Obi-Wan Kenobi? Ewan McGregor’s voiceover and dialogue sound largely the same as they did in previous Star Wars projects, but by listening closely, there are some noticeable changes from how he’s usually portrayed the character. Some of the reasons may be rooted in the actor's real-life, while others are likely meant to be intentional.
In the few voice lines that McGregor’s Obi-Wan has in the Obi-Wan Kenobi trailer, his accent comes across as rougher and more serious than it used to. The scene where he tells Owen that he will one day need to train Luke Skywalker makes this especially apparent. Not counting his small voice cameos in the sequel trilogy, the last time McGregor played Obi-Wan was almost 20 years ago in Star Wars: Episode III - Revenge of the Sith, so it would make sense for his voice to have aged along with him. Not only that, but 10 years are supposed to have gone by in-universe since Revenge of the Sith, meaning that there wouldn’t be a reason for McGregor to make his voice come across as young as it did before.
The age factor also goes into one of the more likely reasons why Obi-Wan’s accent sounds so different: Ewan McGregor could be trying to make himself sound more like Sir Alec Guinness. With Obi-Wan having aged 10 years since his last chronological appearance, he’s now getting much closer to the point where he should look and sound as he did in A New Hope. Based on that, McGregor’s accent in Obi-Wan Kenobi is likely partly intended as a means of helping to bridge the gap between his and Guinness' performances.
Something similar also happened when Obi-Wan appeared in Star Wars Rebels season 3, episode 20, “Twin Suns”. In addition to having a new character model that resembled Guinness’ Obi-Wan, he was given a much older-sounding voice by Stephen Stanton, as opposed to the younger voice James Arnold Taylor portrayed him with in Star Wars: The Clone Wars, so this shows that there was already an intent to make a transition between McGregor’s portrayal and Guinness’ portrayal. The timeline of the episode also helps with this idea; “Twin Suns” was supposed to have taken place in 2 BBY, which would be seven years after Disney+'s Obi-Wan Kenobi, so with such relatively little time between the shows, it would make sense for Obi-Wan to already be aging towards his original portrayal.
All in all, Ewan McGregor’s accent in Obi-Wan Kenobi is likely a combination of age and an intent to connect him to Sir Alec Guinness' version of the character. McGregor’s much older than he was in Revenge of the Sith, so in that regard, it makes sense that he wouldn’t sound exactly the same anymore. In addition, the change in his voice is likely meant to show that Obi-Wan is slowly evolving into the wise old man that Guinness portrayed him as in A New Hope, which could happen relatively soon in the timeline. Whatever the case, there’s more than enough rationality behind McGregor’s current accent for Obi-Wan.