Whilst I do not have papers/journals to support my claim; I am only explaining what I have seen as a novice external auditor.  The role of internal auditing is set to fulfill the requirements of legislation. Otherwise, the role is not effective. This is because the financial directors and CEOs have a lot of influence on the functions of the internal audit departments. I believe the internal audit should be an independent role but that is not the case in developing countries(Nigeria for an instance). In developing countries, to a great extent, the financial directors/CEOs set the boundaries for the internal audit departments. As such, I am of the opinion that the effectiveness of the internal audit is affected.

I suggest a reading that article below. I'm also interested in this topic. We are currently analyzing the relationship of full availability of access to information from T.I. systems of internal control in municipalities in Brazil.

Badara, M.S; S.Z Saidin. Impact of the Effective Internal Control System on the Internal Audit Effectiveness at Local Government Level. Journal of Social and Development Sciences, v.4, n.1, p.16–23., 2013.

Internal audit function is an internal mechanism of corporate governance. Hence, agency theory is probably the closest theory to use in such a study. However, other theories may also be considered depending on the study objectives.

This may help

I believe that Institutional theory might be the best to explain the importance of the internal audit function as it helps to assess the internal controls and accuracy of the day to day transactions. The monitoring role of the internal audit department is essential for an effective functioning of any institution whether in developing or developed countries. Also you can use agency theory given the close relationship between internal auditor and external auditor as external auditor are required to assess the effectiveness of the internal audit function in any organization and determines whether they can rely on some of its tasks when they perform their understanding, testing and assessment of the internal controls as well as their different types of substantive tests.

I suggest using the International Professional Practice Framework published by the Institute of Internal Auditors. You can access a pdf version free at this link from IIA. https://na.theiia.org/standards-guidance/Public%20Documents/IPPF%202013%20English.pdf

Of course, the developing country audit departments may not adhere to all practices, but the IPPF provides a roadmap to aspire to.

As a starting point, the Core Principles can help the developing audit department determine if they are choosing a process that meets the ethos of the Core Principles. See Core Principles here: https://na.theiia.org/standards-guidance/mandatory-guidance/Pages/Core-Principles-for-the-Professional-Practice-of-Internal-Auditing.aspx

Try employ the trio of Policeman theory, Institutional theory, and Agency theory. I believe these theories will help you achieve your set objectives 

I think applying the policeman theory will take the internal audit from its latest aims and development related to adding value for management rather than acting like a policeman or an investigator. I believe that Institutional theory and agency theory characteristics would help to develop the theoretical background needed for the aim of the research

I will use role theory,relational coordination theory and Institutional theory in my research  as I think they are more relevant. than agency theory.