Macbook Air - Mojave - Audacity 2.3.2
recording on USB Codec and built-in mic horrible - staticy, distorted, dropouts… I think I have all the settings the same on this version of audacity as I had on my old Macbook Air (not Mojave I think) - i.e. buffer to 0, 44.1, etc. - went through the FAQs and stuff either didn’t apply or didn’t fix it…
attaching sample without vocals - just recorded “silence” but as you can see it’s very noisy
fixed!! audacity on my old mac had to have buffer to 0… this mac for whatever reason needs to be up to 60…
Thanks for posting the solution. I was going to suggest that you reset the buffer to 100 (the default) and then work from there to find the optimum setting - you beat me to it 
I’ll close this topic as solved.