The algorithm as described in this document works by always looking up filenames in directories (a stat in unix terminology). A good implementation is expected to read the directories once, and do all lookups in memory using that information.
This caching can make it impossible for users to add sounds without having to restart applications. In order to handle this, any implementation that does caching is required to look at the mtime of the toplevel sound theme directories when doing a cache lookup, unless it already did so less than 5 seconds ago. This means that any sound editor or theme installation program may change the mtime of the the toplevel theme directory (such as $XDG_DATA_DIRS/sounds/freedesktop) where it changed the theme to make sure that the new sounds will eventually get used.