I have tried to upload .mp4s to my iPod before, but all it would do is show a screenshot of the video and play the audio. Is there a way to actually watch the video and have the audio play as well?
The 6th Generation iPod Nano does not support video decoding or playback1.
However, it can demux video files and play any supported audio streams in those files2. Instead of the video stream, it displays a single keyframe. This is useful for music videos or podcasts, where you might still want to listen to the audio only, but it's not the same thing as full video playback.
Note that the 5th- and 7th-gen both support video decoding and playback3; its omission from the 6th-gen seems to be an anomaly.
a .Mp4 is not a video format per say, but rather a container. The audio and video content are then encoded with different codecs and placed into the container. What matters is the codec that it is encoded with. Since the video is not playing but the audio is, I'd guess the video codec is not compatible..
I believe the only format that ios devices support is the H.264 codec.
In order to get your video to play, you will need to convert the video file into a format that the ipod can play. There are many software packages available that can do this job for you. Some are free, and some are not.
You might look at handbrake.