Just installed the NEX 4200 in my truck and I am loving it. Installed a rear screen for my 2 year old son and the infrared works great for the DVD but when I switch to content on my USB drive, it will display fine but no audio is routed out of the rear audio port. I can hear sound fine through the speakers but if I try to listen to music, he can't hear any audio like when I'm playing a DVD. Any ideas? Thanks in advance for any help or understanding!