diff --git a/config.py.example b/config.py.example
index 8ed8e28..a7ca318 100644
--- a/config.py.example
+++ b/config.py.example
@@ -2,6 +2,8 @@ import os
 import random
 import string
 from datetime import datetime, timedelta
+from functools import lru_cache
+from mutagen.mp3 import MP3
 
 
 def random_choices(population, k):
@@ -9,23 +11,23 @@ def random_choices(population, k):
     return [random.choice(population) for _ in range(k)]
 
 
-def mp3_sources(data_dir, pattern, kbits):
+def mp3_sources(data_dir, pattern):
     def start_time_for_source_fn(fn):
-        return datetime.strptime(fn, pattern)
+        return datetime.strptime(os.path.basename(fn), pattern)
 
+    @lru_cache(maxsize=None)
     def length_for_source_fn(fn):
-        size = os.stat(os.path.join(data_dir, fn)).st_size
-        length = timedelta(minutes=size / (1000*kbits/8) / 60)
-        return length
+        return timedelta(seconds=MP3(fn).info.length)
 
     def get_sources():
         # Get a sorted list of all source files with start time and length
         sources = []
         for fn in os.listdir(data_dir):
+            fn = os.path.join(data_dir, fn)
             try:
                 start_time = start_time_for_source_fn(fn)
                 length = length_for_source_fn(fn)
-                sources.append({'fn': os.path.join(data_dir, fn), 'start_time': start_time, 'length': length})
+                sources.append({'fn': fn, 'start_time': start_time, 'length': length})
             except ValueError:
                 pass
         return sources
@@ -50,10 +52,9 @@ class Config:
     # [ {"fn": "/path/to/foo-20:01.mp3", "start_time": datetime(...), "length": timedelta(hours=1)} ]
 
     data_dir   = os.environ.get('DATA_DIR') or '/var/tmp/prolefeeder-test-data'
-    kbits      = os.environ.get('KBITS') or 128
     pattern    = 'qfhi-%Y%m%d-%H%M.mp3'
 
     if not os.path.exists(data_dir):
         raise RuntimeError("DATA_DIR does not exist")
 
-    SOURCES = mp3_sources(data_dir, pattern, kbits)
+    SOURCES = mp3_sources(data_dir, pattern)
diff --git a/generate-test-data.py b/generate-test-data.py
index b6b0cdd..cad999b 100644
--- a/generate-test-data.py
+++ b/generate-test-data.py
@@ -10,7 +10,7 @@ def generate_test_file(fn, delta):
         'ffmpeg',
         '-f', 'lavfi',
         '-i', 'sine=frequency=1000:duration=%d' % delta.total_seconds(),
-        '-b:a', '%dk' % Config.kbits,
+        '-b:a', '%dk' % 128,
         fn
     ])
 
diff --git a/prolefeeder.wsgi b/prolefeeder.wsgi
index 7b3c036..b9bdf47 100644
--- a/prolefeeder.wsgi
+++ b/prolefeeder.wsgi
@@ -3,7 +3,7 @@ import os
 def application(req_environ, start_response):
 
     # Work around having no access to Apache SetEnv variables:
-    for k in ['SECRET_KEY', 'DATA_DIR', 'TMP_DIR', 'MAX_LENGTH', 'KBITS']:
+    for k in ['SECRET_KEY', 'DATA_DIR', 'TMP_DIR', 'MAX_LENGTH']:
         if k in req_environ:
             os.environ[k] = req_environ[k]
 
diff --git a/requirements.txt b/requirements.txt
index 3e77d9d..b12ab50 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,3 +3,4 @@ bootstrap-flask
 python-dotenv
 flask-wtf >= 1.0.0
 WTForms >= 3.0.0
+mutagen