I have a movie that at the end of it, I want the action in the final frame to freeze, but the sound to keep looping for eternity.

no. if your sound is attached to a movieclip frame, you'll have very limited
control over it.

if you use the flash sound class, you'll have much more control over your
sound and no issues with timelines (unless you're trying to sync sound and
animation in which case all the issues are solvable but not necessarily
trivial).

I don't need to sync the sound and animation.

Is there any chance you could post some code?

file size should increase by a few bytes when you extend the timeline but add
no new assets to the timeline.



s=new Sound(this);
s.attachSound("yourSoundLinkageID"); //<- assign a linkage id to your sound
in the library
s.start()
s.onSoundComplete=function(){
s.start();
}