Wiki page Overview of third-party security audits - Wiki - Signal Community only contains research papers. There was a TextSecure audit https://www.fox-it.com/be/research-blog/working-with-the-open-technology-fund/ and some evidence that in 2018 Signal paid Doyensec LLC., but no audit reports published.
(TextSecure was audited in 2013 without a report published) and some evidence that in 2018 Signal paid Doyensec LLC., again without any report published. In Signal Reddit members attempted to find security audits before, but have not succeeded: Does the Signal app regularly undergo security audits? : signal and Is there any recent security audit report to look at? : signal
In Signal Technology Foundation - Form 990 for period ending Dec 2018 - Nonprofit Explorer - ProPublica page 8.
“Signal is open source, so our code is regularly scrutinized in addition to regular formal audits.”