A problematic implementation (FB12168694) that doesnt seem to respect changing the ApplicationMusicPlayer.state.playbackRate on iOS 16.4.1 (and possibly other iOS versions). Related discussion here: https://developer.apple.com/forums/thread/702397?answerId=752470022#752470022

AppleMusicPlayer.swift

import MusicKit
import Foundation

public class AppleMusicPlayer {
    private let musicPlayer = ApplicationMusicPlayer.shared

    public init() {}
    
    @Sendable public func prepareToPlay(songID: String) async throws {
        guard let item = await MusicCatalog.song(withID: songID) else {
            throw Unity.instance
        }

        musicPlayer.queue = [item]

        do {
            try await musicPlayer.prepareToPlay()
        } catch {
            throw Unity.instance
        }
    }
    
    @Sendable public func playPause() {
        if musicPlayer.state.playbackStatus == .playing {
            musicPlayer.pause()
        } else {
            Task {
                do {
                    musicPlayer.state.playbackRate = 0.5
                    try await musicPlayer.play()
                    musicPlayer.state.playbackRate = 0.5
                }
                catch {
                    print(error)
                }
            }
        }
    }
}

public enum MusicCatalog {
    public static func song(withID songID: String) async -> Song? {
        guard MusicAuthorization.currentStatus == .authorized else {
            return nil
        }

        let song: Song?

        do {
            let searchResponse = try await MusicCatalogResourceRequest<Song>(
                matching: \.id,
                equalTo: MusicItemID(songID)
            ).response()

            song = searchResponse.items.first!
        } catch {
            print("Music Catalog request failed with error: \(error)")

            song = nil
        }

        return song
    }
}

public enum Unity: Hashable {
    case instance
}

extension Unity: Error {}