The purpose of doing a content audit is to assess the quality and effectiveness of your content against a set of goals--business requirements, customer tasks, performance metrics.
Ultimately, what you are asking is how to speed up something that requires human effort and judgment, but there are ways to automate the data gathering to speed up that cataloging part of the project and get you to the brain work faster. I recommend using a tool, such as the Content Analysis Tool (CAT), from Content Insight, to create the inventory that you need to start your audit. It gives you a complete list of all the pages, images, media files, and documents on the site as well as the links in and out from every page, so you have a base to start from in terms of understanding the quantity of content and the structure. The tool also integrates Google analytics data, so you already have a step in the direction of figuring out what content to focus on.
To scope your actual audit effort, use that analytics data to look at your highest and lowest performing content (highest to figure out what's working and why; lowest to find out what needs to be revised or removed). You can also scope an audit by focusing just on the pages important for your highest-value user tasks--product evaluation through to buyflow, for example. The context for your audit also gives you some ways to limit the effort if you don't have a lot of time--a complete site redesign project is going to be more about looking for site architectural issues and content and interaction models. A rebranding project is going to be more focused on the actual writing and creative and will require a lot more time spent with the content itself.
Another tactic once you have the inventory is to "divide and conquer" the human effort surrounding the audit, for example, an Information Architect could review structure, a Content Strategist could look at content, a Business Analyst reviews functionality, etc.
In a nutshell--know your context and know your goals, and use tools to help you with the stuff the tools do well so you can spend your (and/or your team's) limited time doing the actual analysis.