On digichat's support link, I found the following FAQ. I'm still not sure this explains it, but I'm asking my MIS manager to investigate for me. I'll let you know what I find out.

Why do sounds not play on my system?
Windows 95/98/NT users running versions of Java prior to 1.2 (including all current versions of Microsoft Java, Internet Explorer and Netscape Navigator/Communicator) will not be able to hear sound, due to a bug in these earlier Java releases that caused Windows to crash after repeated playing of sound. Therefore, we have disabled sound for Windows users not running JRE 1.2 or later. You must upgrade to Sun's JRE 1.2 to hear sound on Windows. Because current browsers such as Netscape Navigator/Communicator and Internet Explorer do not use Sun's JRE, sound will not be available to Windows users of the DigiChat Client applet until these companies update their browser software.

In addition, there is a bug in Netscape Communicator 4.5 for Macintosh that prevents it from playing sound from Java applets such as the DigiChat Client. We have notified Netscape of this error, though at present, they have not addressed the problem.