diff --git a/util/audio.py b/util/audio.py
index cad596d7..e2469e8a 100644
--- a/util/audio.py
+++ b/util/audio.py
@@ -92,12 +92,15 @@ class Sample:
         self.audio_type = new_audio_type
 
 
+def _change_audio_type(sample_and_audio_type):
+    sample, audio_type = sample_and_audio_type
+    sample.change_audio_type(audio_type)
+    return sample
+
+
 def change_audio_types(samples, audio_type=AUDIO_TYPE_PCM, processes=None, process_ahead=None):
-    def change_audio_type(sample):
-        sample.change_audio_type(audio_type)
-        return sample
     with LimitingPool(processes=processes, process_ahead=process_ahead) as pool:
-        yield from pool.imap(change_audio_type, samples)
+        yield from pool.imap(_change_audio_type, map(lambda s: (s, audio_type), samples))
 
 
 def read_audio_format_from_wav_file(wav_file):
diff --git a/util/helpers.py b/util/helpers.py
index 73cd9b74..c158066d 100644
--- a/util/helpers.py
+++ b/util/helpers.py
@@ -4,7 +4,7 @@ import time
 import heapq
 import semver
 
-from multiprocessing.dummy import Pool as ThreadPool
+from multiprocessing import Pool
 
 KILO = 1024
 KILOBYTE = 1 * KILO
@@ -83,7 +83,7 @@ class LimitingPool:
         self.process_ahead = os.cpu_count() if process_ahead is None else process_ahead
         self.sleeping_for = sleeping_for
         self.processed = 0
-        self.pool = ThreadPool(processes=processes)
+        self.pool = Pool(processes=processes)
 
     def __enter__(self):
         return self