Android - Why can't music players find my audio files?

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 "special characters" (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 "unidentifyable character", the media scanner most likely has crashed. Which resulted in the media database not being populated, from which follows: The players think "no media files there".

Proposed actions

I'd suggest to either temporarily delete or at least rename all affected files, so that no file with "special characters" 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 ;)

In the stock ROM the Nexus 4 default music player should be "Play Music" with an icon that looks like yellow headphones, never heard of Apollo but it's possible this is something your custom ROM's changed.

Most music players on Android get their "media library" from the built-in media scanner service, this should pick up MP3 files stored in any user-accessible storage location on your phone and add them to the scanner's library. Sometimes the media scanner doesn't "notice" new files you've just added and needs to be told to do a rescan, there are apps on the store that can do this, such as Rescan SD, you can find a selection of them with a search of the Play Store if you run one of those and wait a couple of minutes all of your media should be picked up.

It's unlikely from what you're saying, but it's also possible that a hidden .nomedia file may be sitting in your Music folder, or one of it's parent/child folders. The presence of this file tells the media scanner to ignore the contents of that folder and it's child-folders. If you find that file somewhere in your Music folder's hierarchy, delete it.