Why can't music players find my audio files? On my Nexus 4, there's a folder called /sdcard/Music/ (or just /Music when browsing the phone's storage; Windows 7 calls it Computer\Nexus 4\Internal storage\Music) and I've put a lot of songs in there but the music player doesn't see any of my songs.
Why can one app (the file manager, and e.g. Dropbox) see the files, but not another app (Play Music, Apollo, Jukefox)?

Notes:


LG Nexus 4 16GB, unlocked, with CyanogenMod 10.1
(nightly build 20130212 because nothing more stable is available but there are no crashes.)
The songs are stored in the path format /sdcard/Music/Artist/Album/Song title.mp3 (simply because that's how they're stored on my media drive). 

Computer\Nexus 4\Internal storage\Music\Eagles\On the Border\1 Already Gone.mp3
Computer\Nexus 4\Internal storage\Music\Kashmir\The Good Life\10 Kiss me goodbye.mp3
Computer\Nexus 4\Internal storage\Music\Christina St眉rmer\Freier Fall\1 Geh nicht wenn du kommst.mp3
(yes there's a Chinese character in there and I don't know why; it's a German ü (umlaut u) on my original Windows media drive)

I know that the songs are present and that they work because the CyanogenMod File Manager shows them and I can play them individually. They do exist!
Running Rescan SD as suggested below didn't make the files appear in any of the player apps.
I'm fairly certain there are no .nomedia files: The CyanogenMod File Manager doesn't display files that begin with a . (I renamed an existing TXT file and it promptly disappeared :)) but I used the Root Browser Lite to look through /Music and subfolders -- the only match I found was the one I had renamed, and I have deleted that now.
My Windows pc at work couldn't see the files either, but after unplugging/replugging the USB it now can see+open+play the files. (I've now revised this question to remove the Windows part.)
I'm still mystified by the fact that file-browser apps see the files but music-player apps don't.
Seeing a file name like Computer\Nexus 4\Internal storage\Music\Christina St眉rmer\Freier Fall\1 Geh nicht wenn du kommst.mp3 points to a special problem with &quot;special characters&quot; (such as German Umlauts) which got messed up on data transfer: While Windoze uses its own character set (MS-ANSI or wone of the WIN-12xx, depending on configuration), Android expects UTF-8 -- which is why it's a good custom to use only 7-bit ASCII characters in file names.
Due to this mismatch and the &quot;unidentifyable character&quot;, the media scanner most likely has crashed. Which resulted in the media database not being populated, from which follows: The players think &quot;no media files there&quot;.
Proposed actions
I'd suggest to either temporarily delete or at least rename all affected files, so that no file with &quot;special characters&quot; in its name remains. Then have the media scanner run again (e.g. triggering it with Rescan SD). Once that's finished, I'm pretty sure your media player(s) will find something to play (with).
That being approved, you know what to look for in the future. As said before, I strongly recommend to restict characters in file names to 7-bit ASCII -- that avoids a lot of problems, though some names might look a little strange ;)