What is a current, workable solution which gives an audio out/audio in without having to go via USB, which is known to cause problems since it is shared with other hardware?
My impression is that there is only the Cirrus Logic (ex Wolfson) audio HAT, which looks good in hardware, but is abandoned in software and unusable on Jessie systems due to shoddy code which upstream refuses to accept.
Is there another solution which accepts audio in via I2S or other, non-USB means?
You do not mention which board type you have. On a RPi (not 2) model B (not B+) board you can use the P5 header for I2S as per this article. You will need to do some soldering. Kernel support is available as per Koalo's website, which also features compilation instructions.
For the B+ and 2B, the I2S pins are available as ALT functions on pins 12, 35 and 40 of the GPIO. Check out this document and this thread on raspberrypi.org. This enables I2S. I assume the kernel has already been patched centrally (the document is almost 2 years old) so you should be able to just enable the I2S module.
Then once you have I2S, you would still need to find out how to create audio-in - you'd likely need to solder together your own board using an ADC and a microphone-in or RCA-in plug.
