opus: sample rate is always 48kHz

beets/mediafile.py

@@ -1322,6 +1322,9 @@ class MediaFile(object):
         """The audio's sample rate (an int)."""
         if hasattr(self.mgfile.info, 'sample_rate'):
             return self.mgfile.info.sample_rate
+        elif self.type == 'opus':
+            # Opus is always 48kHz internally.
+            return 48000
         return 0
 
     @property

test/test_mediafile_basic.py

@@ -177,6 +177,7 @@ READ_ONLY_CORRECT_DICTS = {
         'length': 1.0,
         'bitrate': 63216,
         'format': 'Opus',
+        'samplerate': 48000,
         'bitdepth': 0,
         'channels': 1,
     },