Showcasing a concurrency related warning. Discussion: https://chaos.social/@dvk/112438279480546893

UserPreferences.swift

import Foundation
import Observation
import SwiftUI

@MainActor
@Observable final class UserPreferences {
    private class Storage {
        private static let store = UserDefaults.standard
        @AppStorage("isSoundEnabled", store: store) var isSoundEnabled: Bool = true

        init() {}
    }
    
    static let shared = UserPreferences()
    private let storage = Storage()
    
    var isSoundEnabled: Bool {
        didSet {
            storage.isSoundEnabled = isSoundEnabled
        }
    }
    
  
    
    private init() {
        isSoundEnabled = storage.isSoundEnabled
    }
}

struct UserPreferencesKey: EnvironmentKey {
    static let defaultValue: UserPreferences = .shared
}

extension EnvironmentValues {
    var userPreferences: UserPreferences {
        get { self[UserPreferencesKey.self] }
        set { self[UserPreferencesKey.self] = newValue }
    }
}

Ok, here is a potential solution. What I did was transform the instance variable isSoundEnabled into a pure wrapper. This class now has no state. And honestly, I think this is an improvement, because state was previously being duplicated into two places.

import Foundation
import Observation
import SwiftUI

@MainActor
@Observable final class UserPreferences {
	private class Storage {
		private static let store = UserDefaults.standard
		@AppStorage("isSoundEnabled", store: store) var isSoundEnabled: Bool = true
	}

	static nonisolated let shared = UserPreferences()
	private let storage = Storage()

	var isSoundEnabled: Bool {
		get {
			storage.isSoundEnabled
		} set {
			storage.isSoundEnabled = newValue
		}
	}

	private nonisolated init() {

	}
}

struct UserPreferencesKey: EnvironmentKey {
	static let defaultValue: UserPreferences = .shared
}

extension EnvironmentValues {
	var userPreferences: UserPreferences {
		get { self[UserPreferencesKey.self] }
		set { self[UserPreferencesKey.self] = newValue }
	}
}

But, I still think that the root issue is you have too many levels of indirection between your prefs and the UI. I've struggled with AppStorage too, I don't think it is a particularly well-designed API. Here's the link again that might help. I didn't have a particularly easy time implementing this myself, but I think its overall much closer to an idealized version of what is needed here.

https://fatbobman.com/en/posts/appstorage/