diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index 89234b39df..229de4b39a 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -2020,6 +2020,9 @@ def mimetype2ext(mt):
 
     ext = {
         'audio/mp4': 'm4a',
+        # Per RFC 3003, audio/mpeg can be .mp1, .mp2 or .mp3. Here use .mp3 as
+        # it's the most popular one
+        'audio/mpeg': 'mp3',
     }.get(mt)
     if ext is not None:
         return ext