It seems to me the real issue is the auditors currently in Karen-Dawn's program are under the gun to perform ALL their regular tasks in less time in order to devote time to the internal audit process. What I mean is:
If an employee normally turns out 10,000 widgets a week, that same employee is still expected
(by any, or all of, management, supervisor, coworker, himself) to turn out 10,000 widgets a week PLUS perform whatever auditing assignment he is given. If the employee does NOT continue to do his share, then other employees may have to pick up the slack in order to meet the department production quota, creating resentment.
Ultimately, the question is one of management budgeting sufficient time and money for an auditing program, instead of trying to "shoe-horn" it in dribs and drabs.
It seems that Karen-Dawn' management is willing to pay a full-time person, so the question is not one of greedy management, but of lack of recognition (root cause?) of the true reason part-time auditors feel a desire to stay at the regular job full time. The pressure to stay on the job and continue to produce may be subtle or strong and overt.
The key to the root cause may be in the statement
Karen-Dawn said:
We have never mandated internal auditors, our program has been met through various volunteer in addition to your regular duties participants. However, we are going through a real growth/change right now and are not getting the volunteers needed because everyone's tasks are growing/changing.
The basic prejudice pervasive in many organizations is that Quality in general and auditing in particular are cost centers and do not contribute to productivity. Perhaps, in addition to the basics of auditing, ALL employees (and even management) may need to concentrate on the value of a continual improvement program that should be part and parcel of the internal auditing process.
Combining the concept of value to the organization and to the fellow workers together with overtly assuring each auditor he is not a "slacker" for taking time away from a production function to be an auditor, may go a long way toward reversing the trend of auditor drop out. The end result may be a new and stronger appreciation for the role of auditor.

I prefer internal auditors to be part-time on the auditing team and regular employees rather than outside "specialists" hired for the job. I do, however, recognize situations where outside auditors may be necessary, especially in unionized situations.
I do not like the idea of "volunteer" - in my own case, I
assigned folks from multiple disciplines to be part of the auditing team. Over the course of time, everyone in my organization was given the experience of being on an auditing team, even including our full-time janitor and the two part-time receptionists who "work-shared" the full-time reception desk. The unintended benefit was that no one resented a visit from the audit team as an intrusion by the "Quality Qops" since he had already been part of such a team. The intended benefit was that everyone in the organization had a better understanding of the "big picture" of the organization. When we had visits from a customer's auditor or auditing team, I reveled in the looks on their faces when I would ask the receptionist to accompany them through the operation. I enjoyed it even more when they would comment afterward,
"Wow! Your receptionist sure knows a lot about your company." I didn't like the part where I would staff the reception desk until the audit was done.
My suggestion to Karen-Dawn: stick to your guns about part-timers, but consider changing the volunteer aspect as part of discovering the root cause of defections from the audit team.