Apple - Default volume too high for headphones

Take a look at breakaway. You can write a quick applescript to change the volume back and forth when when the headphone is connected/disconnected, and much more

I had the same problem. Setting the volume as low as it could go was still too loud.

Here's a solution that worked for me:

1. Download Loopback
2. Set it up so that you have the following:
3. Sources: Pass-Thru
4. Output Channels: Channels 1&2
5. Monitors: [Select your headphone device]

Now all audio is routed to a virtual device, which outputs to your headphones. This gives you an additional volume slider for more fine grained control, which will allow you to set your headphones to be much quieter than you could normally with the default OSX volume options.