Your tenants generally enjoy what's known as the "right of quiet enjoyment." In part, that means that they deserve to live free from unreasonable intrusions and disturbances -- including excessive noise from other tenants.
However, noise complaints aren't always easy for a landlord to handle. You can't do much about tenants that are overly sensitive to the normal sounds of daily life in an apartment building -- but you also can't afford to ignore a legitimate complaint. Ignoring a valid noise complaint can land you in a legal dispute with the aggrieved tenant.
It's wise to have a clearly defined policy in place for handling noise complaints. Here are some suggestions you can use:
1. Use your lease as a guide.
A well-developed lease will help you avoid these kinds of problems in the first place. Talking to your tenants before they ever move in can help you set expectations that are more likely to be observed.
Focus on the types of noise that need to be curtailed and make sure that you aren't creating any discriminatory policies that can get you into trouble. If you have doubts about the appropriate wording, a consultation with an attorney familiar with real estate law is advisable.
2. Respond immediately.
Even if the noise complaint comes in at an inconvenient time (like the middle of the night), try to respond immediately. That way, you can witness the situation first-hand. That's the best way to determine for yourself if a complaint is really valid.
3. Try direct communication.
If the complaint is valid, a gentle-but-firm conversation with the offending tenant may resolve the issue without the need for further action. If the complaining tenant seems to be oversensitive, you may be able to stop things from getting out of hand by expressing your sympathies while pointing out that apartment living is sometimes a little noisy.
4. Document every interaction.
Make sure that you document your observations and your communications with each tenant involved in a noise complaint -- whether the complaint is justified or not. If it becomes necessary to take legal action, your evidence may come in handy.
Above all, however, do not turn the issue over to your tenants and hope they will work things out. That's likely to only increase the drama -- and your troubles.