Here's a tech note that explains the issue and a possible fix for you:
http://www-1.ibm.com/support/docview.wss?rs=477&context=SSKTXQ&q1=notes+6.5+sametime+sound&uid=swg21167657&loc=en_US&cs=utf-8&lang=en
You should know that although your organization has chosen not to deploy the full-blown 6.5.1 Connect client, it does exist. It has been my experience that the awareness features built into the Notes client are enough for
most end users (who are drawn to the integration w/ their mail file), and we have seen our corp's use of ST chat skyrocket compared to when it was necessary to launch and use the separate client.