Ewan McGregor has a very small role in The Force Awakens, and most fans who have seen the movie probably know what it is by now.
Towards the middle of the movie, Rey experiences a flashback. In this flashback you can hear dialog from Yoda (taken from Empire) and Obi-Wan. One thing Obi-Wan says was never spoken by the character before, and some fans thought they got James Arnold Taylor back in the role. He confirmed on Twitter that Ewan returned for that short line:
It appears as if just the one line is Ewan as the other Obi-Wan dialog in the scene sounds like Sir Alec.