Maybe you could add some more details on what you are trying to do.
If I understand you correctly you want the nRF51 board to sound an alarm either after a certain time, or after a command is sent from some other device?
Do you want to play a sound file from the nRF51, or will a simple beep be enough?
The easiest way to make a beep is to use the app_pwm driver in the SDK.
You will have to set the PWM period according to the beep frequency you want, and then you can simply set the duty cycle to 50% to produce a square wave at that frequency.
The reload time (in us) relates to frequency like this: reload_us = 1000000 / FREQ
To turn off the sound, just set the duty cycle to 0% or 100%.
For an example of how to use the PWM driver, please have a look at the PWM example in the SDK:
\nRF51 SDK_v9.0.0\examples\peripheral\pwm
