Can I switch a video from time lapse mode after it has been shot?

No. Time lapse takes one picture every whatever interval you set. For example, one picture every 10 seconds. Nothing is recorded for the 9 seconds inbetween so no software program can make up what was supposed to be there.

Sorry, no.

Oh my, sounds like you accidentally recorded in "Time Lapse" mode (one of the specialized video features of your camera). Unfortunately, there's no way to convert the already taken video back to "Standard" video with sound, I'm sorry. However, you'll obviously need to set your camera back to standard mode for future videos. Press the "Function" button while in video mode. Then press left/right to select "Standard" video.

This is a really simple problem and easy to solve.  Unfortunately the videos you've recorded cannot be changed back because they have been recorded in Time Lapse mode.  
With the camera switched on in Video Mode turn the control dial on the back (this is the dial to the right of the LCD screen).  This will change the settings from TIME LAPSE to STANDARD.  There are actually 6 different modes but the best one for normal videos is STANDARD.  There is a HIGH RESOLUTION setting,  however these record fewer frames per second (fps) so whilst the quality is better per frame,  there are half the amount of fps so the the movie will look at bit jerky.
It's really easy to accidentally twist the dial when holding the camera,  so always check the icon in the top right of the LCD before recording to make sure its just a yellow camera in a yellow square.  
This should be problem solved for all future recordings.

This is a really simple problem and easy to solve.  Unfortunately the videos you've recorded cannot be changed back because they have been recorded in Time Lapse mode.  
With the camera switched on in Video Mode turn the control dial on the back (this is the dial to the right of the LCD screen).  This will change the settings from TIME LAPSE to STANDARD.  There are actually 6 different modes but the best one for normal videos is STANDARD.  There is a HIGH RESOLUTION setting,  however these record fewer frames per second (fps) so whilst the quality is better per frame,  there are half the amount of fps so the the movie will look at bit jerky.
It's really easy to accidentally twist the dial when holding the camera,  so always check the icon in the top right of the LCD before recording to make sure its just a yellow camera in a yellow square.  
This should be problem solved for all future recording.

This is a really simple problem and easy to solve.  Unfortunately the videos you've recorded cannot be changed back because they have been recorded in Time Lapse mode.  
With the camera switched on in Video Mode turn the control dial on the back (this is the dial to the right of the LCD screen).  This will change the settings from TIME LAPSE to STANDARD.  There are actually 6 different modes but the best one for normal videos is STANDARD.  There is a HIGH RESOLUTION setting,  however these record fewer frames per second (fps) so whilst the quality is better per frame,  there are half the amount of fps so the the movie will look at bit jerky.
It's really easy to accidentally twist the dial when holding the camera,  so always check the icon in the top right of the LCD before recording to make sure its just a yellow camera in a yellow square.  
This should be problem solved for future recordings.

This is a really simple problem and easy to solve.  Unfortunately the videos you've recorded cannot be changed back because they have been recorded in Time Lapse mode.  
With the camera switched on in Video Mode turn the control dial on the back (this is the dial to the right of the LCD screen).  This will change the settings from TIME LAPSE to STANDARD.  There are actually 6 different modes but the best one for normal videos is STANDARD.  There is a HIGH RESOLUTION setting,  however these record fewer frames per second (fps) so whilst the quality is better per frame,  there are half the amount of fps so the the movie will look at bit jerky.
It's really easy to accidentally twist the dial when holding the camera,  so always check the icon in the top right of the LCD before recording to make sure its just a yellow camera in a yellow square.  
This should be problem solved for future recording.

Make sure that you're not on timelapse, it should be standard to do a regular playback.

If anyone can do this, it would take a video production expert that can pull the audio track off and then patch the audio and video track back together. This will require high tech equipment, and quite a bit of expertise to get the two tracks to match. My guess is that this could be done for about $100-200 depending on the expert's rate and actual amount of time they spend on it. Check your yellow pages or the internet for someone in your area that can do this type of video editing.

Good Luck

The TIME LAPSE function makes it possible to shoot a sequence of pictures (such as a flower opening) automatically under pre-set conditions. Open the MODE MENU under the CAMERA tab. Use the arrow pad to toggle up to TIME LAPSE and select ON. On this screen you can select the number of shot (2 to 99) and the time interval between each shot (1 min. to 60 min. in 1 min. increments. Press the OK button and the shooting sequence will begin. It is advised to use the AC Adapter C-7AU for longer sequences. This function is not available in the AUTO or MOVIE Modes.