Thank you very much for your reply Brendan. I tried to see what video codecs I have installed via Windows media player but could not find any menu to get to "Help/About Windows media Player". It seems they have hidden it well. Right clicking the WMV window does not enable me to see that option. You helped me to get on the right track though, and I did a Google search and found it's possible to use third party software like, "Sherlock" to see which codecs I have installed.
According to the "Sherlock" report, the codec, "VBA surface allocator" is broken due to a missing driver because it wasn't uninstalled properly. Well, whatever that means, I was sure that was not the problem.
Anyway I decided to try to import an MP4 file with Videowave again. Today it worked fine! I also tried to import an MP4 audio file using the Roxio sound editor and it worked fine as well! Now, I have been having this problem with the sound editor not being able to import MP4 audio for about a year. (In the past, uninstalling and reinstalling NXT4 did not fix that problem). I am wondering what happened in the last 24 hours that fixed it. It could have been one of 2 things. I was doing some troubleshooting yesterday, and I think I did a repair install of NXT4 from control panel. The other thing is that I went to play an MP4 video sent from a friend's phone and it would not play on my KLite media player. So I downloaded and installed the latest Klite Mega pack and during the install it said that a certain media splitter (Hagaki?) should be removed and replaced by the one from KLite, so I clicked OK. Then this particular video played fine.
So did the "repair install" fix the, "MP4" problem or did installing the KLite Mega codec pack help?
Anyway, its good to be up and running again. It's also encouraging to receive help and support from the forum.
Thanks again Brendan.