You have not experience the nightlife of true Beijinger if you dont visit this place. Beijinger are true singers, they can sing day and night. When I say day and night it means non-stop. We once had a singing sessions among friends that stretches for the whole weekend, from Friday evening all the way to Sunday evening.
Cashbox is the best/most popular place to sing in Beijing. The Chaoyangmeng Branch is the preferred choice. It has the best system around and they run 24 hours with buffet (free of charge) serving every 3 hours.
Usually this place is packed and advance booking is advisable. You might be amused to see the crowd walk-in in the middle of the night at 1am dressing up and ready to sing!! That is a normal sighting.
Dress Code: Casual
If you're in Beijing or Asia for that matter, you have to go and try your hand at KTV. Go with a lot of people. The key is to get over yourself and just give it a try. Most people are terrible but there is always one person that can belt out a few tunes with relative ease. Nobody expects you to be Bono but they do expect you to try. If it helps, they have plenty of liquid courage to get you in the mood.
Dress Code: Depending on the place, dress can range from casual to shorts and flip flops. Don't sweat it though, you're in a private room.