I went to the documentation at the vendor's site https://fe-pi.com/p/support-and-setup . The steps needed to get Linux to recognize the device were correct.
The suggestion that an up to date kernel with a certain patch was correct, use 'sudo rpi-update' to get it.

The real issue was a lack of documentation describing how the device worked and how to use it. The vendor's doc has a screenshot and says "See the alsamixer Linux man page for more detail on how to use alsamixer." True, but that's not enough info!

You really need to go to the NXP web site and download the datasheet ( https://www.nxp.com/products/audio-and-radio/audio-converters/ultra-low-power-audio-codec:SGTL5000 ) and read it. It has a good diagram showing all the flows in to and out of the main audio switch and the various muxes and amplifiers outside of the audio switch. It has a good textual description of what these things do. Once you read it, you can kind of piece together what all the controls are and what they influence.

The vendor's page says "You may also use the mixer provided with Raspbian" but I think you will find this is no longer included with Raspian, all you get is the simple controls at the upper right hand side of the screen. If you want that tool you need to do "sudo apt install -y pimixer".

I feel I was very lucky to find http://www.trinityos.com/HAM/CentosDigitalModes/RPi/rpi2-setup.html#20.setup-snddev

This page went through a lot of very helpful suggestions on how to configure and troubleshoot this device. This is the level of information I would hope the vendor would provide. The author didn't totally understand what was going on (and seems to have not read the NXP datasheet) but overall provided the right kind of info at the right level, IMO.

Their main suggestion is to use the output of a smartphone and a cable with 3.5mm plugs to drive line in, run a linux command 'arecord .... | aplay ...' to record the I2S audio coming into the RPi and send it back via I2S to the Fe-Pi and connect an amplified speaker to line out to hear what is coming out. It also helps to have a headset with mic plugged into the headphone jack at the same time.

In my case the device came up with the line_in signal routed to the headphone jack, totally bypassing the audio switch, and no signal was being routed to i2s_dout so nothing was being sent to the RPi so all 'arecord' commands recorded nothing. Since the switch was bypassed audio on i2s_din was ignored. It was very frustrating. You would think the default would be something more sensible such as 'record from headphone mic, play to headphone speakers' or some such, but nope. I had no idea anything at all was happening till I connected a headset to the headphones jack. It took me a long time to figure out what was going on even having the trintyos link above. Also, even after figuring this out, some times I reboot and find nothing will play even though the alsamixer settings haven't changed. I've found myself having to be prepared to troubleshoot every time I reboot. Hopefully I'll figure out what is causing it to not work some times. So far the only fix has been to reboot. As above, I really think the vendor should provide some troubleshooting steps.

Having said all this, once it works the product is pretty good for what it is, which is an inexpensive way to get stereo line in and line out for the RPi. I verified it can do the max 96000 Hz sampling rate specified in the datasheet which is nice for my application which is ham radio stuff. It takes some time and fiddling to get it set up right. It's far from a turnkey experience, but overall I'm satisfied enough to give it four stars,

Regards,
RDP

Order #163589 Shipped | Tindie
Hi. You can check this house I orders, now I have not yet received. Thank you!

The Fe-Pi V2 board is simply perfect: I know that there are quite a few audio boards for the Raspberries, but this one trumps them all (well, at least those that I've been able to find online :)
* has inputs!! (Line In and Mic),
* very good price,
* very easy to set up and works flawlessly (instructions on author's website),
* I personally like the chip used (SGTL5000), it's the same one as on the Teensy Audio Board and it's possible to talk to it directly via I2C.