I'm not a Linux Master, so take this as my own opinion...
Googling, I've found that :
[ 2.698810] audioinjector-audio soc:sound: snd_soc_register_card failed (-517 )
[ 2.699052] wm8731 1-001a: Assuming static MCLK
[ 2.702201] audioinjector-audio soc:sound: ASoC: CPU DAI (null) not registere d
[ 2.703953] audioinjector-audio soc:sound: snd_soc_register_card failed (-517 )
[ 2.708251] audioinjector-audio soc:sound: wm8731-hifi <-> 3f203000.i2s mappi ng ok <<<<<<<<<<<
this boot messages are normal...the soundcard was succesfully configured after 3 "retries".
In the other hand the message:
[ 9.762220] bcm2835-i2s 3f203000.i2s: I2S SYNC error!
is the one to take care with. I've found many places treating this issue but, so far, I couldn't find any clear solution. Obiously it's a setup issue as the card works out of the box with Raspbian Pixel.
I've also found a nice blog expleaning very clearly how sound/midi works in Linux and how to trobleshoot common issues. I wish it will help you.
I'd try to play some sound using ssh console commands as explained in the blog. Perhaps this will help to isolate what is causing the issue.