Hi Barbara,

On Firefox for the Mac,  use the pull-down menu under "firefox" and choose
Preferences.
Then click the Applications tab and you'll see a list of the plugins you have
set up.  For any particular audio type, you can set it to a specific
application or to "Always ask" if that's what you want.

For Windows, it's basically the same process, except to find it, use the Tools
pull-down menu and click Options, then Applications and it's the same.

Marlene