if you are ever curious about what sound is playing when ... log in to your server with putty, then type: asterisk -R then enter "set verbose 20" and you can watch each event occur. Not helpful with 1000 calls an hour going through the system, but if you are trying to find out things like this, when the system is NOT in use, it's very useful. it will actually give you the name of the file it is playing.
yes, as a rule, you can simply replace like-typed files with another file (replace a gsm with a gsm) without issue.
but if you dont "delete" the old file, you can always put it back without damage. this allows for experimentation