I'm not an ArcaOS user, nor are all OS changes publicly known: append a few seconds of silence to the sound file? Use one of the lesser shutdown utilities to play an own sound with e.g. Rexx (and MCPAPI.DLL), wait for it to finish and - if still needed - pause for a few milliseconds?
If the OS doesn't wait now, then appending silence may be useless. Instead of a test with a prefix or suffix of silence, you'd better try an existing file (e.g. a song) which lasts longer first.