Shoutbox

- If your sound library is public/shared, meaning 'not-private', then everybody who uses your computer will see the sounds in their personal sound library.

- If sounds are in a 'private' library then they will only show up in the custom sound list of the owner of that private library.

- To make a sound library private or not-private/shared you need to set a special registry option (in Messenger Plus! 3) or toggle the option (general>sounds>sound library) in your preferences (in Messenger Plus! Live).

- The private/non-private nature of the whole custom sound library doesn't have anything todo with the 'private' flag of individual sounds.

- Custom sounds are stored locally.

In order to also have them on another computer you either need to export them from the first computer and import them on that other computer via a 'soundpack'.
Or you need to alter the registry on that other computer; you don't physically need to copy the sounds (as each time you use one of the new sounds for the first time it first needs to be downloaded, which will be done automatically).