So as it turns out, it was Ewan McGregor who voiced Obi Wan’s one line in the ‘vision’ sequence:
http://www.ew.com/article/2015/12/20/jj-abrams-reveals-obi-wan-and-yoda-are-star-wars-force-awakens
Never suspected that, it sounded like the same Guiness-soundalike from the Rouge One teaser. Or maybe that was also Ewan? In either case he’s certainly did a very good impression.
If you missed it, the line was “Rey! These are your first steps.” Which implies that Obi-Wan spoke to Rey in the past, to the contrary of her memory. So maybe Obi did train Rey a little? And that’s why she’s so strong in the force all at once? Sounds about right.
More likely Luke trained her a bit before she was left on Jakku, but she has little memory of it. The stress of her situation and having touched the lightsaber brings some of it back.