My camera breaks long recordings into 4 gb chunks. When I combine them end to end in MS13 Platinum onto the timeline, there is a break in video and sound at juncture points. I have tried bringing them in directly off card using device explorer, but does not help.
What usually works for me is to use the DOS binary copy & concatenate to combine the separate MTS files into a single file. In a DOS prompt window you can type something like this:
In most all cases this will seamlessly join the files. There's always a chance that your camera does something funky that prevents this from happening, but it's worth a try.
Actually, i usually name the combined file with a .mpg extension instead of .mts because Vegas 9 complains less about opening .mpg.
You didn't say what kind of camera you have - but if it is a Panasonic G-series there will always be about a half-second delay at the start of every clip for the audio track. That's why I always use a 2-second fade-in for all tracks.