I'm running into the same situation with version 5.01, which is why I usually stick with version 4.22. Windows Media Player 12 won't play the video I just put together with an alternate music score. I've tried:
Using both the K-Lite Basic and Standard codec packs, but neither of them fixed the issue.
Using the K-Lite Codec Tweak Tool to reset anything that might be set wrong in the OS registry regarding the codecs or anything DirectDraw related to no avail.
Uninstalling Windows Media Player from the OS (Windows 7 Pro), rebooted, reinstalled WMP, rebooted, went through the usual 1st time WMP use dialog boxes, saw MP4 as being a format WMP understood, but it will only play the audio and not the video.
What will work is using Divx media player, iTunes built-in media player, and the media player used by the K-Lite Standard codec pack.
None of my experience is mission critical to me since I use VideoPad as intermittent hobby software rather than something dependable.
One of the best places to get older versions of the software is FileHippo.com. The program is listed under VideoPad Video Editor. Version 4.0 is the oldest version available.
The one thing I haven't done yet is to do separate installs of version 5.01 and 4.22 in separate folders, if possible, in order to check the file folder contents, file types, time date stamps, etc. In a previous 3.x version, there was a missing file issue that kept me from being able to convert video to MP4 H.264 video code standard. So, maybe there's something similar happening or something different altogether.
The audio, but no video with WMP 12 has been an on going issue for years and not just an NCH Videopad issue. Since my current video is working with the other media players, there must be something file, programming code, or registry wise that's causing WMP 12 a problem. Again, it's an annoyance for me, but probably very aggravating if you're trying to do an important and meaningful project.
You can try doing the things I've mentioned to see if they resolve your problem, but don't hold your breath.