I'm not sure if this will only work in Sage 100 Standard as opposed to Advanced and Premium), but ProvideX can play a beep with the PRINT 'BEEP' function. This is how you would reference it in script:

oScript.Execute("PRINT "+"'BEEP'"+CHR(40)+CHR(34)+"*.Default,w"+CHR(34)+")"

Place it before your MessageBox line. Also this is the default Windows sound event. There are more in the registry you can reference.

Also a few years ago, I posted some How To scripts including one on how to identify a duplicate customer name and play a sound using the Windows Text to Speech API (one of the Accessibility features). I also mention how to play the sound from the workstation if you're running Advanced or Premium -  How to Prevent Duplicate Customer Names

Hope this helps.

I remember the scripting class where you had us use that text to speech command.