** solved! **
Just figured this out after a healthy day of head-bashing. There are very nice and clear instructions found at the bottom of this thread. In case anyone else has this problem and is totally new to this, I installed qjackctl using ‘sudo apt-get install qjackctl’. Then I started qjackctl, and set everything up as in thread. This still wasn’t working for me- turns out I had to use the command ‘killall jackd’ to make sure my old jack servers were gone, and then set everything up 
Hi all,
I’ve been using sonic pi on my raspberry pi, and it’s been great! I have a script in which I record a live audio sample and play it back some time later. My code works just fine on my laptop. To use it on the pi, I got a USB mic. It works records as expected when I use it outside of Sonic Pi. However, I can’t get it to work in Sonic Pi using synth :sound_in. I think this might be because it’s a separate soundcard? I know from the multichannel audio section of the tutorial that I can specify the input number- can i use this to select the correct recording device? Or does anyone have suggestion on how to do this?
Many thanks for any help! 
EDIT: after some more research I found this reply about using qjackctl to choose the soundcard. Before I try to dive too deep into this: does this seem like the way to solve my problem, and is it possible to have Sonic Pi use two different soundcards, one for capture and one for playback? (since i’ll still be using the default raspberry pi for playback). Thanks for any help!!
EDIT 2: So, I got qjackctl and have been messing around, not really knowing what I’m doing. In the setup section, I went to “Advanced” and changed my input device from “default” to my USB Audio device (USB Audio hw:1,0). This doesn’t seem to have done anything though- in the ‘Connections’ window, known of the available connections correspond to my USB audio device. Is there anything I need to do in Sonic Pi to register these settings, maybe? Thanks again!