There is an interesting thread in which these issues are addressed:
A Process Audit is where the company s procedures are validated.
Processes are sub-parts of a system. As such, they are typically a part of a system audit.
Process audits are almost always a part of a larger system(s) audit. This is not to say that process audits are only performed as a part of a larger systems or registration audit. An internal audit may indicate the need to perform a specific process audit, for example.
Almost always, one or more other process(es) will interact with any given process. One very important issue to consider is the effectiveness of communications between systems and/or processes.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions