Let's start with what you've got going on. It seems to me that you have 2 copies of some AIF files, and one copy of an identical song as an M4A. But what you really want is just the AIF files that are on the M: Drive. If that's true, it's pretty easy to remove everything else and just have the library contain M: files only.
The real power of JRiver MC is in it's database. The Library is (sorta kinda) the database. If you ignore the Library, you're ignoring 90% of the power of MC. I highly advise that you use it.
That being said, I am told that you can browse purely by file in MC and not use the Library at all. I don't use the Windows version of MC and I don't have that feature, so I can't tell you how to do it or what the considerations are. Someone else surely can.
To clean your library up so it only shows M: files, it's two major steps.
A. Tell MC to not auto import from anywhere except the folder(s) on the M: drive you want imported.
1. Tools > Options > Library & Folders > Configure Auto Import
2. In the dialog that comes up remove all drives and folders that you don't want to use in MC. That will keep it from looking there for any new files.
B. Remove all of the files from MC from locations other than M: .
1. Go to a view where you can easily see the file names. I like Audio > Files .
2. Look in the pane at the bottom, on the right side for the file name. You may have to scroll to the right. You should see a mixture of files from all of your drive locations. Take note of the number of files displayed at the very bottom.
3. In the search box up at the top, type: M:
4. Make sure you now see all of your files from M: and note the number of files. Should be smaller than the last number you saw.
5. Now in search just add a minus sign. Like this: -M:
6. You should now see everything *except* for files on the M: drive.
7. Select all the files down below. Just click any file then go to Edit > Select All
8. Press delete to remove the files.
9. In the dialog that comes up, take the first option: Remove from Media Library Only. That way it won't touch the files. They will still be on your disk(s). Just removed from the Library.
10. Erase the search term in the search box. All of your files from M: should show up and nothing else.
That should do it. You should probably make a Library Backup before you start. Just in case. File > Library > Back Up Library...
Good luck and let us know if we can help any further.
Brian.