External Drive, can't empty trash, rm sees a file, but ls -la doesn't
The problem appears to be caused by a file named 03 - Ēlusion.mp3 located within the directory named folder.

Because Terminal.app is unable to render diacritical marks in file names--well, it is, but that's beyond the scope of providing the simplest solution for you--it hides its failure by hiding the filename (something unheard of by me before now; perhaps its High Sierra's changes to /.file, /.volfs and 64-bit inodes? Wait-- never mind; your edit to your question tell me I misunderstood you.) Anyway, the file's existence is known, along with the ironical contention by the Finder that it doesn't exist. Obviously, it does. Here's how to change that:

First, determine the file's inode number. In Terminal.app, cd to the directory "folder" and issue this command: ls -i *.mp3

Copy the number string of the inode provided in the left-hand column of the response, which will be something like

12345678  03 - E ̄lusion.mp3

--and put it into this command, which will rename it to something the terminal can render correctly:

find . -inum 12345678 -exec mv {} deletemenow \;

--giving you the file "deletemenow" in the folder "folder," both of which you may dispose of in whatever way best suits your fancy.