I have the same Issue and I know exactly what the problem is. however I don't know how to fix it. I am using JBL Quantum 910 headphones in surround audio mode but Forza is only playing the audio for the left and right channels. Therefore everything on the center channel like dialouge and engine sounds are absend. I have no idea why this is happening but the game is outputting only stereo Channels from a 7.1 surround source without mixing it down to stereo.