Hi...(Long ramble now...
)
I have no idea what might be causing this effect as I can't get it to happen. (Sorry! ) However, when you say that you recorded the video and later added the sound, what exactly do you mean by "recorded the video" as normally you would simply save the project and then resume work on it later....Is this what you mean?
I can outline what I do when I use images and video together with separate sound files as this may be of some help to you.
I use jpeg images either from my video camera or a separate still camera. These have a different aspect ratio to video, so if I am mixing the images with film I generally crop them to the same aspect ratio as the video (1:1.77) so as to avoid borders when they play. (Note that you can do this to the images once added to the sequence line by using the zoom function as you can set 4:3 or 16:9 . You simply set 16:9 but no zoom- i.e. start and end frames set to the same size.) However a lot depends on the image content as to whether cropping is possible. In some cases I use VP generated stills directly from the sequence clips in which case the ratio is the same as the video which is H .264 1280 x 720 AVC HD format , normally converted from MPEG4 to MPEG2. I usually create .wav sound files in a separate program for commentary etc. and use MP3 files for sound effects.
For a project similar to what you are describing,I would proceed as follows:
Open VP and "Add Media" to the media list. These will be my images; sound files and video clips.
In order to see what images etc. I am adding to the sequence line I switch into storyboard mode. The contents take up less horizontal room on the screen as they appear as thumbnails and relevant info like clip/image duration is shown. Check the "Options" tab however to verify that the default duration values etc. are near to what you want. In my case it is set to 6 seconds.
Following your scheme and assuming I have created a static welcome jpg image, I would click on its file name on the media list and add it to the sequence.(I usually drag it and drop it where I want it to go). This image will (or should) appear with a duration value of 6 seconds displayed in the box beneath the thumbnail. I then repeat the procedure with the remaining clips/images building up the sequence across the screen and adding any transitions etc.. For each video clip there is usually an accompanying soundtrack but these are shown the same length as the parent clip as they are, of course, linked.
At this point I might play the sequence in the right hand preview pane to check all seems well and the images etc. are playing for the durations indicated.If it seems OK I save a copy of the project and do this at regular intervals.
Now, again, following your scheme for the separate sound clip. I would drag the sound file (your audio welcome) down to the first soundtrack and drop it under the image thumbnail against which I want it to play. Note that In storyboard mode the sound clips are visually compressed or expanded depending on their length as the sequence scale is linear and not time scaled,so this sound clip will not necessarily be on the same longitudinal scale as the video linked soundtrack.
So now switch back to timeline mode and adjust the length of the static welcome image in the left hand preview pane at the bottom until it matches your audio track. If there is any extraneous noise in the audio, I would either drag the ends of the clip in or out to eliminate it or possibly click the clip and then use the volume controls in the left hand preview to null the start and fade in to the actual voice. (You click the yellow audio level line and then drag it up or down to change the level at a particular point. As you can create numerous points it's easy to mute sections of the audio clip or create fades in/out etc..
If I am satisfied with the final sequence I usually save it to a DVD-RW disc to check how its works.
Click "save Movie"......Disc....Make DVD Movie... Make my other selections-(PAL-16:9 )...OK....Continue without chapters etc.
I then simply replay the DVD to check it is as I expect. The drawback with this is that for a long movie the rendering can take hours and it would need to be repeated on a standard DVD once I was happy with the test.
For me this procedure works OK. The images play for their correct length with the commentary exactly where I put it. Nothing is missed or truncated and the images dissolve properly between clips/images.If you follow this as a test and your jpegs still don't play although the preview in VP is OK then....don't know 
BTW. You could create your static text welcome screen using a blank frame and a text overlay.
Nat