curs_beep.3x man page
beep, flash — curses bell and screen flash routines
The beep and flash routines are used to alert the terminal user. The routine beep sounds an audible alarm on the terminal, if possible; otherwise it flashes the screen (visible bell). The routine flash flashes the screen, and if that is not possible, sounds the alert. If neither alert is possible, nothing happens. Nearly all terminals have an audible alert (bell or beep), but only some can flash the screen.
These routines return OK if they succeed in beeping or flashing, ERR otherwise.
SVr4's beep and flash routines always returned OK, so it was not possible to tell when the beep or flash failed.
These functions are described in the XSI Curses standard, Issue 4. Like SVr4, it specifies that they always return OK.