Warner Home Video released on the day of Superman/Batman: Apocalypse's official release, i.e. September 28, 2010, three videos to promote the direct-to-video animated feature. Two clips feature voice actor Summer Glau discussing the role of Supergirl, and the other is a clip from the actual animated feature.
Bonus: here's an excerpt from an interview with Superman/Batman: Apocalypse director Lauren Montgomery for CBR, where she talks about what it was like having Summer Glau as part of the cast as well as how she ended up with the role.
Interviewer: How did you land on Summer Glau and what did she bring to the role of Supergirl?
Lauren Montgomery: Summer was really interesting. I think she was Bruce [Timm]’s idea. He was the one who put her name in the hat as far as a possible Supergirl voice option. And she wanted to come do it, which was awesome. And when she came in, she was really nervous. I don’t think she’d ever done any voiceover before, so we were a little frightened. It was like, “Oh my gosh, is she going to be able to do it?”
We went through a rehearsal and she was still kind of nervous and still holding back but as soon as we started recording, it was like she was a pro. It was like night and day. There was nervous Summer and there was Summer who was just doing her job. She was just acting. And she did an amazing job. I was sweating it a little at first, but as soon as she started performing for real on the record, she was amazing. She did such a good job. I was really happy with her performance.