I know this is an old post, but it's the first that comes up in a web search for turning off sounds in the SAP GUI.
Here's how to turn off sounds in the newer SAP GUI 730:
1. Click the Customize Local Layout icon in the toolbar, then Options...
2. Expand "Interaction Design", click "Sound Settings"
3. Uncheck "Activate audio signal"
4. Click Apply, then OK
Hope this helps someone.
Glenn