Well Macromedia hasn't been around for quite awhile, and it all works just like
you want, so stop over-thinking it. It also sounds like you should do some of
the basic tutorials on working with the timeline and the playhead.

You know what frame the sound ends at because the blue wave frame ends. If it
doesn't end you haven't reached the end and will need to add additional blank
frames.

How could it scale the sound to fit? If it scaled it then you would either
have chipmunks or chopped off sound or something else undesireable. You've got
2m6s of sound, that is how long the sound will be. At 20 frames per second that
is 2520 frames -- more or less. If you change the frame rate of the document
the length of the blue line will change, because again it all works just like
you are asking.

As for making them taller, I think the timeline only has normal and short
choices for the height of the timelines. So if you aren't already on short it
doesn't get any taller.