According to Entertainment Weekly, the voices of Frank Oz (Yoda), Ewan McGregor and Alec Guiness (both Obi-Wan Kenobi in the prequels and originals, respectively) can be heard during one pivotal scene in the new film.
Apparently during Rey’s vision that comes to her after finding the Skywalker lightsaber, great Jedis of the past speak to her with new voiceover… well, sort of. Unfortunately, Guiness passed away in 2000 but they were able to work around that. According to Abrams, they were able to take a clip of the original Obi-Wan saying “afraid” and clip it so he would say “Rey.” Additionally, they had the second Obi-Wan (or first, if you’re going in saga order, I suppose) McGregor record the line as well, with Abrams gushing, “We asked Ewan McGregor to come in and do the line. And he was awesome and we were very grateful. He was incredibly sweet and handsome, and all that stuff. Then he rode off on his motorcycle. Literally the coolest voice over actor ever.” The final line is “Rey, you’ve taken your first steps.”
As for Master Yoda, Abrams said that Oz graciously lent his voice, saying “He was incredibly generous and came into Bad Robot, where we had a recording area, and he was doing Yoda, saying a number of lines we gave to him. This whole experience has been one outrageous moment after another. Just watching Frank Oz, you look at him and talk to him and his voice is very deep. I don’t know why I would have thought he sounded like Miss Piggy!”
Finally, another returning character is one we don’t see much of in The Force Awakens… but, it’s not who you think. R2-D2 is dormant the majority of the film but springs to life near the conclusion. Abrams has also explained this decision, saying, “While it may seem, you know, completely lucky and an easy way out, at that point in the movie, when you’ve lost a person, desperately, and somebody you hopefully care about is unconscious, you want someone to return.”