It would be great if Sonarr could mark downloads as failed and replace them based on the MediaInfo once the download has completed.
Specifically, if stereo copies are downloaded for a show where surround sound is preferred. Not all release groups indicate the audio channel information in the release name so keyword restrictions do not help.
This check should happen before importing the episode and reject it if the current episode file does meet the criteria. If the current file does not, (or there is none) import the new file anyway, but mark it to be replaced by a new download as soon as one becomes available. The user could configure if they would rather replace a lower quality surround sound episode with a higher quality stereo copy or prioritize surround sound over resolution/source.
An awesome add on to that would be if Sonarr sees that downloads from a particular release group are always stereo (say after 3 times, or whatever number the user configures), it then blocks that release group on that show (or all shows where surround sound is wanted, also user configurable). This would save on wasted bandwidth usage in the future.