Play sound from Apple Watch speaker

This is now possible as of watchOS 2 using WKAudioFilePlayer or WKInterfaceMovie.

NSURL *assetURL = [[NSBundle mainBundle] URLForResource:@"file" withExtension:@"wav"];

WKAudioFilePlayer example:

WKAudioFileAsset *asset = [WKAudioFileAsset assetWithURL:assetURL];
WKAudioFilePlayerItem *playerItem = [WKAudioFilePlayerItem playerItemWithAsset:asset];
WKAudioFilePlayer *audioFilePlayer = [WKAudioFilePlayer playerWithPlayerItem:playerItem];
[audioFilePlayer play];

WKInterfaceMovie example:

[self presentMediaPlayerControllerWithURL:assetURL options:nil completion:nil];

It is not possible to play sound out of the Apple Watch's speaker, but you can trigger playing a sound file on the iPhone, here is thread about it

import AVFoundation
var player: AVAudioPlayer?

if let path = Bundle.main.path(forResource: "siren", ofType: "wav") {

        let fileUrl = URL(fileURLWithPath: path)

        do{
            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback)
            try AVAudioSession.sharedInstance().setActive(true)

            player = try AVAudioPlayer(contentsOf: fileUrl)

            guard let player = player else { return }

            player.play()

        }
        catch
        {

        }

    }

I've used this to play a custom sound from the apple watch(4.3) speaker and worked just fine. Don't forget to set the audio file target membership to the watch kit.