Hey again, I am working on a small case for my Zynthian inspired by the OP-1. I want to include a small speaker in the case and have that be the default output unless there are headphones connected to the sound card. I am wondering how I would be able to do this, more specifically what audio card I should use and how I would set this up. Currently I have the Zynscreen v1.5 along with the encoders, I also have a cheap clone of the PiFi Dac+ v2.0 I bought from Ali Express (which I have not had any success with). Any help would be greatly appreciated thanks 
As @riban mentions I have done a little in this regard.
as with all my threads it’s a bit of an epic . . .
But it works very well.
My machine zynthian-amp2.local also runs with a hifiberry amp2 card and just … works.
The issues are you don’t get an ausio output, everything goes straight to the speakers and you dont have an overall volume control as we haven’t worked out quite how to implement that yet, but as a device for playing MIDI to a couple of speakers it’s great. I’ve lent it to completely non technical people who have just used it as a piano, and no complaints.
Your best option is probably purchase a small class amplifier board and run the speakers and headphones from that wired on to the normal zynthian output connectors.
I have found this board [WM8960 Hi-Fi Sound Card HAT For Raspberry Pi] and it looks rather promising. How would I go about installing the drivers and get it working on the Zynthian? Also what gpio pins do the official Zynthian display use as I am worried about it interfering with the sound card hat. Thanks
Look at the schematic:
https://wiki.zynthian.org/images/e/ee/ZynScreen_v15_sch.svg
so for the display itself: pins 15,19,21,23,18,22,24,26
you can get inspiration from here:
A friend of mine has been inspired by my zynthian so he’s decided to build something similar. He’s opted for the WM8960. I had a look at the details in the above link on github, but am not sure how to proceed. Does a driver need to be installed via command line? Or is there a similar card that can be used, from the web interface list?
No I don’t think so. It looks like the driver has to be compiled during installation process and need some systemd startup script.
I would recommand to Make some installation test on a spare sdcard with Raspberry pi Os on it before trying on a zynthian sdcard.
AudioInjector use WM8731 in some of their soundcards. You could try those from the list. In fact it is worth trying each option to see if it works before looking at compiling your own. Of course you could have chosen a supported chipset. 
Thanks for the suggestions! I suspect compiling a driver is beyond my quite limited capabilities in this field. We got as far as trying a few different options from the list, but without success. I don’t think we tried all the audioinjector options, though so that may be worth revisiting. I did try to persuade said friend to go for a card that’s obviously supported but they found a WM8960 card going cheap.
In the end we got the zynth working via an external USB device they remembered they had. Now there’s another very happy zynth owner joined the club 
So often the cause of hours of lost time. Not always cheap if it ends up not working!
Indeed! And in this case, not even used 
At least it was a cheap waste of time 