OK, first up, there exists a fairly cheap WiFi "SD Card reader", that is fairly infamous. The Z-Sun card reader.
https://www.amazon.com/Memory-Reader-MicroSD-iPhone-Android/dp/B00U70J9UA/ref=sr_1_4?ie=UTF8&qid=1510671542&sr=8-4&keywords=zsun+wifi+card+readerLong story short, this thing's security is a complete and total joke, with telnet left open with a hard coded username and password direct to a root shell. Dont buy this thing for any reason other than as a very inexpensive SoC for doing fun projects with. It is also fully supported by OpenWRT, and the wifi chip in it supports multiple wifi connections simultaneously. See this nifty little page about it.
https://wiki.hackerspace.pl/projects:zsun-wifi-card-readerIts main claims to fame are: Fairly powerful for its size, has a microSD card reader, pretty neat Wifi capabilities, USB port powered, VERY TINY.
Ok-- now, armed with that-- let's see another bit of fun hobbyist kit: The micro USB hub board.
https://www.tindie.com/products/mux/nanohub-tiny-usb-hub-for-hacking-projects/Taken with the teardown pics on the hackerspace page for the zsun, you can clearly see how these two things were almost made for each other. I strongly suspect that if you unsolder the card reader from the zsun mainboard, then attach this hub board instead, you could then attach the card reader's header to port 1 of the hub, and another USB device to port 2 on the hub, and both could be controlled by the zsun SoC, simultaneously.
With me so far? Good.
Now, let's look at one last, final bit of hardware kit. Syba USB audio dongle.
https://mightyohm.com/blog/2008/10/inside-the-syba-sd-cm-uaud-usb-stereo-audio-adapter/You can clearly see where I am going with this. Unsolder the USB connector, then directly solder the board to the mini hub's remaining USB header, and stack it on top of the SDCard reader.
The zsun should now have both an SDcard slot, and a USB audio device attached.
Now the software:
On linux, there are kernel drivers for this audio device, and those drivers are available for openwrt.
https://wiki.openwrt.org/doc/howto/usb.audioNext, we install the Music Player Daemon
https://wiki.openwrt.org/doc/howto/usb.audio#mpdthen lighttpd (and php!, and MySQL)
https://wiki.openwrt.org/doc/howto/http.lighttpdhttps://wiki.openwrt.org/doc/howto/phphttps://jimtech.wordpress.com/2016/06/19/openwrt-mysql-server-install/then finally, put netjukebox on the lighttpd server.
http://www.netjukebox.nl/install.phpGive it a sexy 3d printed case to close it all up in, and you have local storage for MP3/OGG files, a wifi ethernet card to connect to the local network (even the internet for internet radio, since MPD supports doing that, and netjukebox will let you add such internet streams to the play list!), and you can hook it up to the AUX stereo input of your home theater system, and control the whole thing using the web portal.