Many of my old videos were uploaded without audio, they only get few views.
But now I want to add audio to them, Can I ?
but Don't want the youtube free soundtrack, I want to upload an audio file from my computer to the youtube video.

Also, there are some stupeflix videos on my channel, recently, one video got the copyright notice, dont understand why.
Stupeflix, is a website that enable you upload like 10 images and add the audio in stupeflix audio category, then export the stupeflix video directly on youtube. But don't know why they don't clame this: "we don't get the audio copyright to publish on youtube", did they?
This leads to my video get copyright notice, I'm somehow angry with stupeflix that leads me to copyright notice video.
Also it's ridiculous that youtube always says "you needn't do anything to this copyright notice video", but they can simply close your account one day for this copyright notice video :S. Why can't they change their words as "this is very serious problem, please delete this video or change the audio immediately."

And do you know, does youtube's free soundtracks is always free? lifelong free?
If I change to free youtube soundtracks, "can I be relaxed that the free youtube soundtracks won't get any copyright in the future" ?

The question is too long, but please help. Thank you!