campfire/Assets/Scripts/CasetteProgress.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class CasetteProgress : MonoBehaviour {
    public static int CasetteIndex;
    public static int CasetteCount;
    public static bool AllCasettesPlayed {
        get {
            return CasetteIndex >= CasetteCount;
        }
    }
    public static AudioSource CurrentlyPlaying;
    public Text Display;

    private void Start() {
        CasetteIndex = 0;
        CasetteCount = 0;
    }

    private void Update() {
        if (CurrentlyPlaying != null) {
            Display.text = CurrentlyPlaying.isPlaying ? "Playing diary..." : "";
        }
    }

    public static int PlayNext() {
        if (!AllCasettesPlayed) {
            int Index = CasetteIndex;
            CasetteIndex++;
            return Index;
        } else {
            return -1;
        }
    }
}