This is a Raspberry Pi Media/Musicplayer case, which also can be used for general purpose (for example building a thin client).
It is designed to use it with a Raspberry Pi 3 and a IQaudio PiDAC+ (http://iqaudio.co.uk/audio/8-pi-dac-0712411999643.html ; the small plastic ring on top of the cinch connectors has to be removed to close the cover. This 5mm space would totally wasted in my optinion.).
Other soundcards can probably be installed if the dimensions and the connector positions are nearly the same.
If you want to use the same parts, here is my bill of materials, including article numbers of the German electronic parts store "Conrad Electronik": https://www.dropbox.com/s/zka5diwjgptpeqg/BOM_Conrad_Electronic.pdf?dl=0

The USB connectors are common panel mounted USB connectors with male plugs.

In order to redirect the HDD activity to a front mounted LED, you have to add a "Device Tree Overlay" (please google what this is). This is pretty simple, if you use a RasPi 3 - Just add the following line to your "config.txt":

dtoverlay=pi3-act-led,gpio=xxx

Replace xxx with the pin the LED is connected to.

To use the encoder to control the volume, there is a simple piece of C code available from iqaudio.co.uk. However, it needs to be slightly modified depending on your music player distribution or audio config in general (soundcard device name).
Compile it (the wiringpi library needs to be installed!) and make it starting while boot (take a look at the PiDAC+ manual!).

Happy printing! :)