Timer1b will always be set up so that the compare-match function is triggered when the audio should start or stop. This means that the timer1b compare-match ISR will need to compute the next relevant time and reload the 1b compare register. Timer 1a is used to generate the high speed interrupts necessary for the squarewave frequency. Buttons 0-3, when individually pushed, produce four different calls, according to the table below. Once started, the call repeats indefinitely, until the next button push.
All times are in milliSeconds.
Frequencies are in Hz. Demo this program to a staff member. Show that all specifications are met by verifying times and frequencies with an oscilloscope. The scheme you used to detect the pushbutton state. How you generated the freqencies and an estimate of how accurate they are.