To remove music or a sound effect from your Flash movie, open the .fla's Library panel and delete the sound file(s).
To replace a sound file the process is considerably more involved. First you'll want to put your new sound file into the .fla: Choose File > Import > Import to Library from Flash's top menu.
In the Import window that opens, browse to where you saved the new sound file that you wish to use.
Select your sound file and click Open to import the sound file to your Flash movie's Library.
Next, locate the Layer that holds the original sound for your movie.
In Fig.1 below, Layer 13 shows the waveform for music in the Timeline.
Click on the sound's keyframe (the small circle). In Fig.1 above the sound's keyframe is in frame 2.
The Properties Panel will display the sound file's information:
Click on the dropdown arrow and select the name of your new sound file that you imported:
Your new sound has now replaced the original.
Note: If your new sound file is of different length than the original, or you wish it to loop a different number of times, you may click the Edit button in the Properties Panel to further finetune your new sound file. (The Edit button and Loop options are pictured in Fig. 2 above.)
To replace a sound effect, such as in a button symbol, the process is the same as described above, but you may have to search a little more to locate the original sound that you want to replace.
In this example the sound effect is in the Over state of a button symbol named "invis":
And in this example below, from a Full Flash template, the sound effect that will play when the viewer's mouse touches the "Contacts" button is actually not in the same symbol as the word "Contacts", but is inside a symbol called Symbol 21, in a Layer called "buttons":
So be prepared for something of a "treasure hunt" when attempting to locate certain sound effects, and remember the Properties panel (shown as inset above) can tell you the selected symbol's name. You can then access it through the Library panel -- should you become lost wandering about in different timelines and would rather go straight to the source.