Cool plugin, but I would not make two versions of the plugin. Writing in the description, that if the user wants the standard sounds, that he doesn't need to upload the new sound files, so you can use it like this on your plugin:
PHP Code:
public plugin_precache()
{
if (file_exists(XXX))
{
precache_sound(XXX)
}
else
{
precache_sound(XXX)
has_standard_sounds = 1
}
}
And then check somewhere the variable to play either the standard or the custom sounds.