Adjust the Volume Depending on the Output Device in OS X
If you connect your Mac to multiple audio devices, you’ve probably noticed that volume affects different speakers differently. How-To Geek reminds us that you can adjust the volume depending on which device you are connected to.
If you go to System Preferences> Sound and select the Output tab, you can set the volume for each device. When you connect to this device, be it a Bluetooth speaker, your internal speakers, or an Airplay device, the volume automatically changes to whatever you choose. If you often switch between speakers, this is a very handy little trick.
How to Adjust Volume Settings for Specific Audio Devices and Sound Effects in OS X | How-To Geek