This is not a bug but rather by design. When the STREAM_VOICE_CALL is in progress, all other STREAM will duck at reduced volume. The user experience is such that during phone call, all other audio stream that didn't mute (music, ring, notification) will be played at lower volume.
AudioPolicy behavior like this can be introduced by OS, Manufacturer, or Carrier when customizing the Android OS. For this particular case, I have seen it across all Note2 (added by Samsung), but haven't seen it on S3. If you can, please share the carrier of the S3 you have.