Videos

It supposedly supports Mp4/3GP, but when I put videos on it (in Mp4 format), I can only hear the sound, there is no actual video. Does anyone know exact specs that the videos must adhere to? I have a video converter and have tried different resolutions etc, but nothing seems to work.

Any help would be appreciated. Or if anyone knows of a video converter that will convert to 3GP so I could try that out..

I can't find anywhere that lists the supported video codecs. I would just go with 3GP if you can. Try Bink & Smacker to see if you can convert the files you want to a format the phone supports.

.mp4 is only a container class (like .avi) it can use different types of codecs internally (compression algorithms.)

What the Xenon recognizes is the Quicktime MPEG-4 part 2 "codec" which gets confused with .mp4, not the same thing.  Spec writers get lazy and just say .mp4 or MPEG-4 which can refer to multiple things.
 
Your .mp4 file that only plays sound is probably a .H264 "mpeg-4" codec which is MPEG-4 part 10.  There are different codecs for audio and video, it probably uses an audio codec that the phone recognizes.

There are many different free encoders which do a decent job, some let you pick a lot of options some just have a single "mobile" profile.  If you look for iPod converters you will usually get the option to make an .H264 or Mpeg-4 file.. chose the Mpeg-4 and it should work.  Another encoder I've seen asks if you want the "old" or "new" version of .mp4, chose the "old" version.