Vendor documentation from the manufacturer of the physical hardware is the best source of best practices for securing the BIOS.

Explanation
Integrity is focused on the trustworthiness of data as well as the prevention of unauthorized modification or tampering of it. A prime consideration for maintaining integrity is an emphasis on the change management and configuration management aspects of operations, so that all modifications are predictable, tracked, logged, and verified, whether they are performed by actual human users or systems processes and scripts.

Explanation
Auditing involves reports and evidence that show user activity, compliance with controls and regulations, the systems and processes that run and what they do, as well as information and data access and modification records. A cloud environment adds additional complexity to traditional audits because the cloud customer will not have the same level of access to systems and data as they would in a traditional data center.