Someone made a compilation of academic reviews and blogposts here: https://community.signalusers.org/t/wiki-overview-of-third-party-security-audits/13243 but none of them seem to be real security audit reports, ex. compare with real security audits to Delta Chat: https://delta.chat/en/help#security-audits
So it looks like the protocol was audited, but I don’t know about the app or servers. https://www.pindrop.com/article/audit-signal-protocol-finds-secure-trustworthy/