My Canon PowerShot SD300 was recording sound perfectly a few days ago. Now whenever I record a video and play it back there is no sound except for static. I have resetted the settings to factory defaults but it still does not work. Please help me.

Sounds like the edge of the tape has been damaged. The edge of the tape has the speed encoded so the playback will know what speed to play the tape. Look at the edge of the tape for wrinkles. If present, throw away the tape so you won't use it for an important recording.

Perhaps the microphone was muted while you were making the original recording? Have you tried playing back a new recording with the microphone active?

As far as I know there is no way to adjust the microphone's sensitivity on this model. Make sure you are recording in a relatively quiet environment as these cameras' microphones tend to pick up everything and can generate a lot of background noise in your recordings. If you still feel like you need to shout into the mic to hear anything then you should look into getting your camera service by an authorized Canon service center. Hope this helps!

Hiram
Photo Tech Repair Service
110 East 13th Street
New York, NY 10003
212-673-8400

check your menu where you have disabled the sounds.
In menu you can enable and disable all sounds.

This sounds like dirty capstan and heads, your recordings will suffer like this, when did you last run a cleaning products in your camera?

I'm sorry to tell you, but the PowerShot A410 model does not have a microphone to record voice. Of all the A400 models, only the A410 and A420 were made this way. For some unknown reason, Canon decided to not include micorphones wiith these two cameras, thus they can only record video, but no voice.

Here is some info from fileinfo.com :
Video thumbnails are often created on digital video cameras and digital cameras that record video; THM files may or may not be transferred to the computer depending on what software is used to import the video; each THM file shares the same filename as the video clip it represents, i.e. "clip001.avi" and clip001.thm."

Are you sure you have not recorded the video in time-lapse mode? Could be worth checking. Once in the Video setting, you can use the thumb ring to change video mode - one of these modes is normal and one is time lapse (there are five or six others).

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.

Is the menory card full?