What about the random player? Does it take songs from all the playlists or only from the selected one?
In my software you can randomly play a certain Genre or randomly play the Top10/25/50/100 songs played so far. Now, my software does not support playing Top10/25/50/100 songs from a particular genre and it does not support randomly playing songs from a certain playlist.
Basically, you can add any songs you want into a playlist. My software displays all the playlists which exist (and songs within the currently selected playlist), so the user can then scroll through the playlist to select which songs they want to hear.
Is any app able to crossfade two songs?
I did not implement any crossfading effects. It is more like a traditional jukebox.
@unclet: can I modify the names of the different genres (for example in "Album") and can I lock single genres so they even cannot be seen in the album list?
You can create any "Genre" names you want (ex: "Blues", "Rap", "Wacky Songs", "Not For Old People", etc ...) You can then choose a particular genre so that only those albums associated with that genre are displayed. You can then also hide the "Select Genre" button (via the Options/Party Lock page) so no one can change the genre to something else.
I could live with one large library with all the singles and albums and make albums for singles like "Singles Letter A" to "Singles Letter Z"....but what happens if the software reads a "album"-directory "Singles Letter A" and it holds 200 songs?
I believe my software can read up to 999 songs per albums, although it has been a long time so if you try it, you can let me know

I know I support 3 digit track numbers so I am assuming this means I can handle up to 999 songs per album.
And it should be possible to lock the albums so only the singles can be seen.
If you create a Genre called "Singles", then you can select the "Singles" genre to be displayed only.