Internally we already can distinguish whether a track is audio or video track. This information is also useful in tagging, e.g. to place videos elsewhere.
The videotools plugin provides helper functions $is_video() / $is_audio() for this (https://github.com/metabrainz/picard-plugins/blob/2.0/plugins/videotools/script.py)
I would like to have these functions moved into Picard core.