PROBLEM: While creating a Playlist using the " (selected files) " method, scrolling through the 12000+ songs in the list, I select many non-adjacent songs. If I preview a song I automatically lose all of the previously selected songs. Since these songs were non-adjacent recreating the list is terribly time consuming.
REQUEST: A way to preview a song without losing the already selected songs in the list.
Perhaps a Playlist settings option that enables the Right Click Menu property that won't deselect files. And, maybe even a toggle button in the toolbar.
I realize this would force the notion of mouse placement defining the preview song. Perhaps in this mode the right click uses a different color for the highlight row.
With 12000+ songs (only 1000 or so are in albums) I create Playlists manually. I regularly want to preview a song before including it in the playlist. I don't have a good workaround. The clumsy way is to scroll through the collection using windows explorer, but then I'm scrolling through 2 lists at once using 2 monitors.
A few other options that spring to mind
Alternate 1: perhaps a toggle button in the toolbar that when enabled changes the right click behavior to that of song selection for the purpose of creating a playlist. That is, while the feature is enabled, right click on a song selects it for being added to the playlist, then double left clicking would still preview a song.
Alternate 2: perhaps adding a Playlist section to the Right Click menu where the action of adding the song to a Playlist is listed. Perhaps up to last 5 playlists are listed or create a new one. This way scrolling through the list and previewing the songs remains the same we just would have the option to add a song to a playlist using the right click menu. Options for the listed playlists in the right click menu would be controlled using the Playlist section of Tools.
Thank you for the consideration.
Great program. 
AFAI understand selections, functions are always applied to all selected objects - so a "preview" would probably play all selected files. At least it behaves like this on my Windows computer. The selection, stays, BTW.
Is this Mac behaviour?
You could create a set of actions that fills a user-defined field like WHICH_PLAYLIST:
For the first playlist:
Action of the type "Format value" for WHICH_PLAYLIST
Format string: Playlist 1[; %which_playlist%]
For the second playlist:
Action of the type "Format value" for WHICH_PLAYLIST
Format string: Playlist 2[; %which_playlist%]
and so on. (The square brackets would add the previous contents of WHICH_PLAYLIST if there is any).
This would make it possible for a single track to be part of several playlists.
Once you have finished the basic selection, use the filter
%which_playlist% HAS "Playlist 2"
to reduce the display to all the files that have been tagged to be part of Playlist 2.
Then create Playlist 2 with the proper name.
I just want to select songs (mostly non-adjacent) from the list and be able to preview any non-selected song without losing the selections. The selected files playlist creation works great.
Currently, right click Play option plays the already selected songs. At least some of the time, often the music player freezes, but this not an mp3tag problem. I want to preview a song NOT already selected to decide if I want to include it or not.
You're suggestion for my Alt2 would work, except (1) {if I understand correctly} now my music files get new attributes, which I don't want (maybe others will), (2) the approach is playlists assigned to music files, I prefer the other way round, and (3) I realize this Alt2 suggestion is a half step beyond the scope of mp3tag, which is to address tags.
Thank you for the consideration.
Great program.
I do not think that there is a single program around that executes a function on an unselected file.
Have you tried your workflow e.g. in the Windows Explorer: select a couple of files, then right-click on an unselected file to play it?
In my explorer the selected files are immediately unselected the moment I right-click on a previously unselected file to play it.
So I think the preview of an unselected file is not feasible.
I have no idea what is more important for you: the preview or the creation of playlists (with the help of additional data).
Anyway: until the feature is implemented the modified alternative 2 is my suggestion.
BTW: you can delete the extra field as soon as you have finished the generation of the playlist.
This current behaviour is consistent with how most file browsers work.
Perhaps a slight modification to your workflow will aid in the meantime. As you are selecting many non-adjacent files to add to a playlist and only stopping to preview a track you aren't sure about, simply add the current selections to the playlist first. Then you can easily preview the single track, and continue on selecting after that. Repeat if you wish to preview another track before you are finished.
I can't think this would add any more time than your requested feature.
Just phantasizing on my part:
Perhaps the "hover" event could be (mis-)used to start a player after a certain time. This would avoid a real selection.
I am not sure if every player has a stealth mode to simply play a track without a full gui.