android schedule task to execute at specific time daily

Solution 1:

To fire alarm everyday, at 21:32

private AlarmManager alarmMgr;
private PendingIntent alarmIntent;
...
alarmMgr = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
Intent intent = new Intent(context, AlarmReceiver.class);
alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);

// Set the alarm to start at 21:32 PM
Calendar calendar = Calendar.getInstance();
calendar.setTimeInMillis(System.currentTimeMillis());
calendar.set(Calendar.HOUR_OF_DAY, 21);
calendar.set(Calendar.MINUTE, 32);

// setRepeating() lets you specify a precise custom interval--in this case,
// 1 day
alarmMgr.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(),
        AlarmManager.INTERVAL_DAY, alarmIntent);

In this case, AlarmReceiver is the Broadcast Receiver and it already has a context, so you can directly set the ringer mode to silent from the Broadcast Receiver without starting an activity.

@Override
    public void onReceive(Context context, Intent intent) {
       AudioManager am = (AudioManager)getSystemService(Context.AUDIO_SERVICE);   
       am.setRingerMode(AudioManager.RINGER_MODE_SILENT);
}  

Please be sure to add these permissions :

<uses-permission android:name="android.permission.WRITE_SETTINGS" ></uses-permission>
<uses-permission android:name="android.permission.CHANGE_CONFIGURATION" ></uses-permission>
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" ></uses-permission>

Solution 2:

If Shivam Verma's answer does not work for you, make sure you add the receiver to your manifest:

<receiver android:name=".receivers.MyReceiver" />