Address comments

Author: NicolasHug

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -1178,7 +1178,14 @@ void SingleStreamDecoder::convertAudioAVFrameToFrameOutputOnCPU(
   int desiredSampleRate =
       streamInfo.audioStreamOptions.sampleRate.value_or(sourceSampleRate);
 
-  int sourceNumChannels = getNumChannels(srcAVFrame);
+  int sourceNumChannels = getNumChannels(streamInfo.codecContext);
+  TORCH_CHECK(
+      sourceNumChannels == getNumChannels(srcAVFrame),
+      "The Frame has ",
+      getNumChannels(srcAVFrame),
+      " channels, expected ",
+      sourceNumChannels,
+      ".");
   int desiredNumChannels =
       streamInfo.audioStreamOptions.numChannels.value_or(sourceNumChannels);
 

src/torchcodec/decoders/_audio_decoder.py

@@ -39,9 +39,9 @@ class AudioDecoder:
             Note that this index is absolute across all media types. If left unspecified, then
             the :term:`best stream` is used.
         sample_rate (int, optional): The desired output sample rate of the decoded samples.
-            By default, the samples are returned in their original sample rate.
+            By default, the sample rate of the source is used.
         num_channels (int, optional): The desired number of channels of the decoded samples.
-            By default, the original number of channels is used.
+            By default, the number of channels of the source is used.
 
     Attributes:
         metadata (AudioStreamMetadata): Metadata of the audio stream.