Sound gradually turns off by itself.

Hi, I have a sony vaio VGN-NR310E laptop. Today i was on youtube watching a video when i realized my sound gradually becoming lower until it was muted. I hadn't touched anything. I clicked the sound icon and moved the bar up to increase the volume and the bar would gradually move down lowering the volume by itself until it was muted again. has anyone ever faced this issue before? and how should i fix it?

 

Is your mouse behaving erratically?

Can you disable the touchpad on your computer and test with an external mouse?

 

thank you for the reply.
The sound went back to normal after awhile and i haven't faced the issue again yet. The cursor wasn't behaving erratically during the sound problem but there has been times before where it did not respond correctly to the touchpad. Sorry, I can't test it because i don't have an external mouse to use.