Recursive search project folder hierarchy?

I know this is over a year old question, but thought I'd answer for archive sake.

You could use Directory.GetFiles() with the SearchOption.AllDirectories and have it return a list of all files with filter. Here's a snip I used for finding all audio files I wanted that existed in Resources:

 // imports...(I hate it when people don't post imports)
 using UnityEngine;
 using UnityEditor;
 using UnityEditorInternal;
 using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using UnityEngine.Audio;
 
 // ... later in code
 protected List<string> GetFiles()
     {
         List<string> fileList = new List<string>();
 
         try
         {
             string path = Application.dataPath;
 
             IEnumerable<string> files = Directory.GetFiles(path + "/Resources", "*.*", SearchOption.AllDirectories)
                 .Where(s => s.EndsWith(".mp3") || s.EndsWith(".wav") || s.EndsWith(".aif") || s.EndsWith(".ogg"));
 
             foreach (string f in files)
             {
                 fileList.Add(f);
             }
 
 
             return fileList;
         }
         catch (UnauthorizedAccessException UAEx)
         {
             Console.WriteLine(UAEx.Message);
         }
         catch (PathTooLongException PathEx)
         {
             Console.WriteLine(PathEx.Message);
         }
 
         return null;
     }

Hope that helps,

John