Noise complaints are notoriously difficult to deal with. What one person considers an unsuitable level of noise may be seemingly quiet to someone else. Other times, residents simply don't like each other and will use any excuse to complain about the other. As a property owner and landlord, you need to try and run a mediation between people who are at odds with each other over something like noise. The question is, how exactly can you do that when you aren't even living there?
The first thing you should do is make sure to include a clause in the lease about noise. This may just be what the town law is regarding volume levels, but at least there will be something for your tenants to refer to. If it can be proven that one tenant is in breach of this clause, then things are much easier to decide. While minor noises must be expected, especially in a multilevel apartment building, sometimes people are excessively loud. Ask for the tenant bringing the complaint to the table for some form of proof, such as a recording, and see if the complaint continues or not. Sometimes people just need to talk something out and honestly noise complaints don't need to be any different in this regard.
You may experience a problem if a resident has an unusual work schedule, such as night shift or if they are a nurse, or are on call for varying hours. Many times people in these professions will sleep at odd hours to get any sleep they can. It can be very difficult for your residents who follow these schedules to get sleep if others use this time to make a large amount of noise. Asking people to be courteous is just about the only thing you can do in regards to noise levels. The best you can do is hope others will be understanding.