So... to play multiple files at once, each file needs to be marked with a different "play type". You mark files by right-clicking on a song.
Playing Multiple Files
So basically, you can play one of each type, at the same time. Right now we have
Trying to play two musics at the same time, will end the first music. But trying to play music and an audiobook, will play both.
You can also end any playing song, by clicking on the active song-list and deleting the song from the active-list.