I can talk at least a little about sample rate. @samaaron might know a little more about it, but from, what I understand, Sonic Pi’s default sample rates are just whatever SuperCollider (scsynth) boots with.
You can alter the values that scsynth uses, but how you do so will depend on whether you are prepared to work with a self-compiled bleeding edge version of Sonic Pi, wait for the next beta or release version, or use an existing release version.
If you are prepared to build a bleeding edge version from source, or wait for the next beta or release version, then there have been recent changes in the ruby code to customise the boot up settings that are sent to scsynth.
The relevant change is here: https://github.com/samaaron/sonic-pi/commit/41ceedc0508de59b62064083262dda451f5a48cc.
If you have a version with this change in it, then the file to edit is
settings.json - which is in your
.sonic-pi folder. That folder will be in your home directory unless you have specified that it be stored somewhere else using the
SONIC_PI_HOME environment variable.
Happy to talk about editing
settings.json more if that is the path you decide to go down.
On the other hand, if you wish to use a current release version, then the best option is to just use the Audio MIDI Setup app on your Mac to edit the sample rate of your default input and output devices before booting Sonic Pi. (Which need to be identical if you wish to use sound input in Sonic Pi on a Mac).
Bit rate I am not so sure about - all I can really tell you about that is that for example when I use the recording function in Sonic Pi on my Mac here, it outputs a wav file in 32 bit float format.