Addperiodictimeobserver Is Not Called Every Millisecond

CMTimeGetSeconds duration and addPeriodicTimeObserver doesn't equal at the end

Since the times are coming from different sources, there might be slightly discrepancies on the 'delta' of milliseconds, hence you should round them doing so:

 if floor(currentSeconds) == floor(durationSeconds) {
    self.handleAudioFinishPlaying()
 }

In iOS AVPlayer, addPeriodicTimeObserverForInterval seems to be missing

Check this func addPeriodicTimeObserver(forInterval interval: CMTime,
queue: DispatchQueue?,
using block: @escaping (CMTime) -> Void) -> Any

It is in the documents also for example check this code snippet

let timeObserverToken = player.addPeriodicTimeObserver(forInterval: interval, queue: DispatchQueue.main) { [unowned self] time in 
}

Referenced from here

AVMutableAudioMixInputParameters setVolume atTime not working only with kCMTimeZero

You should do something like this:

- (IBAction)maxVolButtonPressed:(id)sender {
    [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(setVolume:) userInfo:nil repeats:NO];
}

- (void)setVolume:(id)sender{
    [audioInputParams1 setVolume:1 atTime:kCMTimeZero];
    [self applyAudioMix];
}

In first method you are saying: wait for 1 second and then call method setVolume:.

How to use AVPlayer addBoundaryTimeObserver?

As per the docs

times - An array of NSValue objects containing CMTime values representing the times at which to invoke block.

You need to create appropriate CMTime structs then create an array of NSValue objects:

let cmtime = CMTime(seconds: 2.18, preferredTimescale: 100)
let cmtimevalue = NSValue(time: cmtime)
let cmtimevalueArray = [cmtimevalue]

Be aware that the seconds for this initialiser is Double and timescale is CMTimeScale aka Int32

Also your arrayOfBeats is not an Array, it's a Dictionary and items in dictionaries are NOT ordered. You probably won't get the order you were looking for.

You might be better served with an array of tuples.

let arrayOfBeats = [("2.18", 3), ("3.38", 5), ("3.63", 6)]

Converting your String seconds value back to Double is your (trivial) problem.

Trying to understand CMTime

A CMTime struct represents a length of time that is stored as rational number (see CMTime Reference). CMTime has a value and a timescale field, and represents the time value/timescale seconds .

CMTimeMake is a function that returns a CMTime structure, for example:

CMTime t1 = CMTimeMake(1, 10); // 1/10 second = 0.1 second
CMTime t2 = CMTimeMake(2, 1);  // 2 seconds
CMTime t3 = CMTimeMake(3, 4);  // 3/4 second = 0.75 second
CMTime t4 = CMTimeMake(6, 8);  // 6/8 second = 0.75 second

The last two time values t3 and t4 represent the same time value, therefore

CMTimeCompare(t3, t4) == 0

If you set the videoMinFrameDuration of a AVCaptureSession is does not make a difference if you set

connection.videoMinFrameDuration = CMTimeMake(1, 20); // or
connection.videoMinFrameDuration = CMTimeMake(2, 40);

In both cases the minimum time interval between frames is set to 1/20 = 0.05 seconds.

How do I get current playing time and total play time in AVPlayer?

You can access currently played item by using currentItem property:

AVPlayerItem *currentItem = yourAVPlayer.currentItem;

Then you can easily get the requested time values

CMTime duration = currentItem.duration; //total time
CMTime currentTime = currentItem.currentTime; //playing time

Swift 5:

if let currentItem = player.currentItem {
    let duration = CMTimeGetSeconds(currentItem.duration)
    let currentTime = CMTimeGetSeconds(currentItem.currentTime())

    print("Duration: \(duration) s")
    print("Current time: \(currentTime) s")
}