These may seem like a small things but when you are dealing with so many tapes, the little things add up. As for what to put on the tape, start with a grabber. Remember, you are trying to catch the attention of someone looking at lots of other work. Unless you are applying for a specific type job, like a sports photographer, you should cover the bases and include hard news, spot news and a good feature or two. Usually an employer is looking for a certain kind of photographer.
Some shops place a high priority on experience and technical proficiency.
I was looking for creativity and an ability to work closely with subjects. I was particularly impressed by photographers who found new ways to do the same old stories. On a resume tape, quality is far more important than quantity. Do not send anything for which you have to apologize or make excuses. It is a good idea to pre-mix your audio to one channel.