1. Do everything you'd normally do in an interview - Well, you've got the clothes down, what's next? The answer is the same as if you were interviewing for a job in the business world. Make sure you look nice, print out two (or more) copies of your resume to put in a plain folder, take something to write with/on (an 8.5" X 11" notepad and a couple of pens preferably), and get there 15 minutes early. Doing all this might seem like overkill, but it tells your potential future employer "I'm prepared for this job, and I'm gonna get it."
2. You're not applying to be President - Your job is important, but it's not super official. That being said, don't campaign. Your tone of voice when talking to your interviewer should be just like your dress: casual, but not sloppy. Titles like "sir" or "madam" probably will get you laughed at, while titles like "dude" or even worse "yo" will cost you your job. Address the interviewer however he introduces himself to you. As an example, if I were interviewing you, you'd want to call me Alden.
3. Relax - During the interview, you're probably going to be quizzed. Whoever is conducting the interview might want to see how much you know going in. This isn't necessarily a "get it right or you don't get the job" kind of thing. Since an internship is about you learning the ropes when it comes to tech jobs, they expect you not to know much. Having a working knowledge of how to use equipment is definitely a plus. Not knowing that you don't give a $2000 ribbon microphone phantom power might be a deal-breaker, because it'll cost the company some money. On the other side, whether or not you know what the three prongs on an XLR cable do probably won't matter too much, because you'll learn that as you work. Know that being quizzed is part of the interview process for a tech job, so study up, but don't stress.
4. Ask questions and get the tour - Nothing shows that you want the job more than asking a bunch of intelligent questions. Asking the interviewer flat out "may I have a tour of the gear?" is like asking your girlfriend "would you like this diamond necklace?" It tells the interviewer that you have a genuine interest in the work they do, and that you are excited to see such fancy stuff. When you're looking at the equipment, feel free to be a gearhead. Avoid saying things like "yeah, we have a bunch of these at school", but instead say "I've had some experience with these, and I love how they sound." When the conversation is broken down to basics, you should show genuine excitement for working with the gear, but don't make it seem commonplace. Lots of places have API pre-amps, for example, so making note of that is unnecessary. Lastly, if you have a genuine question about a piece of gear, now is the time to ask. Don't know what makes a convolution reverb different than a plate reverb? Ask.
As always, if there's something you think I missed, or you have any questions about this post, feel free to add it in the comments below. Good luck on the interview!