END OF LIFE
Thanks to the wonderful folks at npm, in npm v10.2+, after 6 years, npm audit no longer requires a lockfile!
Therefore, you should no longer use aud. Instead, use npx npm@'>=10.2' audit --production.
aud 
Use npx aud instead of npm audit, whether you have a lockfile or not!
It's a great idea to run npm audit in CI; it ensures that you don't unknowingly have vulnerabilities in your dep graph.
Unfortunately, it doesn't work without a lockfile and only apps should have lockfiles. It also requires npm v6 or above.
Now, instead of npm audit, you can run npx aud! If your repo has a lockfile, it will just run npm audit; if it does not, it will use npm-lockfile to copy your package.json and your currently configured audit level (npm config get audit-level) to a temp dir that has the proper version of npm installed, it will use npm install --package-lock-only to create a temporary lockfile, and it will run npm audit there. On exit, all the temp dirs will get cleaned up.
aud fix without a lockfile present will throw npm audit's normal "no lockfile" error, since there's no way to preserve fixes to transitive dependencies.