Offline Music from USB

Hello!

I've searched the last 2 weeks for Audio Players for my Raspberry Pi, but yet haven't found one that fits my purpose.
In theory its really simple:
My hardware includes a Pi (2 or 3) and a cheap amplifier plus speakers, there is no need for a good hifi experience.
I just want an offline player, that starts very quickly (because no special modules/services) and auto play all audio data on the USB stick I insert, without any command.
Its headless and the only control is a powerbutton on the cable. I choose PiCorePlayer because you can just power off without caring about a shutdown.

The installation was fast and simple, I had to connect my LAN cable to it as WiFi doesn't work out of the box and looked for the settings.
I disabled the splash screen and set an USB mount point to try, but unfortunately nothing happens.

So I have to ask first, is it possible to achieve my mentioned goal or does PiCorePlayer only work when you are online?
Or is there another minimalistic program for RPi that does the things I need better?

Thanks in advance!