I've found a workaround for this that works for me. I used returns 1/2 on the Helix to receive the audio from the 3.5mm out of my iPhone using a stereo Y cable with a right and left 1/4 inch. Then set the input for path 2 on the helix patch to receive the audio on input return 1/2. I was then able to set the audio volume on the iTunes track and play along through headphones using the phones volume control on the Helix to control the overall volume.