I had the same problem with my Pioneer AVH-P4050DVD
Radio and CD worked fine
DVD would play and I could hear the audio but no video
USB and iPod. It would connect and I could brouse, select and play songs but I would not get any audio even though I could see the track playing, finnishing and moving on to the next one (very frustrating).
Front Aux (Aux 1). I could turn this port on to audio, select it and plus an audio sorce in but no audio would come out.
The Pioneer AVH-P4050DVD is odd and different from most other pioneeres in that it has two earth leads that both need to be grounded (2 in addition to the handbreak wire). I had insalled my unit using a toyota - pioneer wireing adaptor I bought from Repco. This adaptor only grounded the wire coming from the corner of the plug (opposite the battery wire). The second earth is located becide the first earth, opposite the ignition wire. If you google pioneer 16 pin wiring diagram it might call this pin BGSENS (pin 13 in the attached photo). With the wireing harness I had this pin was not connected to anything. Once I grounded the second earth (soldered directly to the first earth) everything worked.
Good luck. Hope that helps.