GarageBand not responding to volume controls - deafening!

I bought a headphone splitter for my macbook - just a standard 1/4 inch short cable to two 1/4-inch inputs so I can use two sets of headphones with GarageBand and Logic.

And now garageband is deafening!!! and the volume control on the macbook doesn't work.

I had to bring the gain of the project way down, to -23dB, just to tolerate it.

Is there a way to remedy this? I'm not seeing anything on the audio settings.