Is it possible to get an audit of all folders and documents in a Sharepoint document library automatically?
You can enable auditing, which does exactly what you want for that library. After it’s enabled you can generate reports under Site Settings -> Site Collection Administration -> Audit log reports.
You may also get help from SharePoint auditing tool: https://www.lepide.com/lepideauditor/sharepoint-auditing.html
Correct. However OP would like an entire audit for the entire site collection.