Under the Triggers tab for the flag tag, set up an Active On trigger with the Beep() function as the action (see function reference PDF for proper syntax). This will cause the beep to occur for a specified time every time to tag becomes true.
If you want the beeping to continue indefinitely until the tag switches off, you'd probably have to add some complex code to the On Tick action in the Global Actions. Something like this:
if (Tag1) Beep(60,500);
When executed "On Tick" (once per second), this will cause a 60Hz 500msec beep to occur if Tag1 is true.