As a test, you could drag & drop some of the problem files to MC's Playing Now list. (For example, if a complete album is missing from audio views, arrange the Windows Explorer and MC windows side by side, select the album files in Windows Explorer and drag them to MC's empty Playing Now list.
If MC can list the files in Playing Now you can check the import status and try playback.
MC has a neat trick that can show if the files in a Playing Now list and the "Drives & Directories > Explorer..." section in MC's selection tree are already imported to the library (i.e. to MC's internal database). You can add the "Image (small icon)" column to a list view and see the import status. The icon's color will show this. More info:
http://yabb.jriver.com/interact/index.php?topic=59513.msg402349#msg402349http://yabb.jriver.com/interact/index.php?topic=59513.msg402379#msg402379http://yabb.jriver.com/interact/index.php?topic=59513.msg427787#msg427787If the files show up as imported and playback works, I wonder if the problem could still be caused by missing or incorrect file tags. You can recheck the tags in Playing Now.
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Personally, I would do this the other way round, because I would like to preserve my audio playlists. I would start with the audio library and import the video files.
If you just import the video files as new library entries, the video tags can be preserved if MC has stored them to sidecar files, but there is an alternative method that can preserve also all statistical data and the possible existing non-standard cover art links. You can select the video files in the video library and expert an MPL playlist that contains all file metadata (File > Export Playlist... select the MPL playlist format.) Then load your audio library and import the MPL playlist (File > Import Playlist...).
You can even preserve the original "Date Imported" values if you import the same MPL playlist once again. The first MPL import will import the video files as new library entries and thus also the Date Imported values will be new. The second MPL import will only update metadata, including the Date Imported values, because the files are already present in the database.
EDIT
Effectively this MPL export/import method is very close to "merging libraries", although with some limitations. It cannot merge playlists and for some reason it cannot correctly import the CD/DVD database section that stores metadata for physical optical discs.