Which version of the program are you using and on what operating system?
Which model of camcorder are these MTS files coming from? Is your camcorder recording your audio as stereo or 5.1 audio?
When you added the video to your project, did you wait for it to completely conform (as indicated at the bottom of the program) before you started editing it?
Try clearing the file from your Project Assets and re-importing it into your project. Wait until the file is completely conformed before you start editing it.
It's adobe elements 12. Windows 8 OS.
It's from a canon hdr-cx430v.Recording 5.1ch sound.
I waited for it to conform. I deleted & re-imported it.
When I play it separately, from the stored folder, the sound is there.
So when you waited for it to conform, it worked, right? (You did delete it completely from your project and reimported it, right? You didn't just remove it from your timeline.)
Do you see a waveform on the audio track?
No, it did not work.
The waveform stops a minute before the end - it goes flat.
How long is this particular video file? Something is apparently interrupting its conformation.