The audit is not for you. Closed source software is audited all the time, but the results of those audits are generally confidential. This is about finding security bugs, not deliberate backdoors.
The key with this is who do you trust. Sure, open source can be audited by everyone, but is it? You can’t audit all the code you use yourself, even if you have the skills, it’s simply too much. So you still need to trust another person or company, it really doesn’t change the equation that much.