Broadly, Ext Aud do not have a responsibility to prevent or detect fraud, but should detect material fraud as that is a material misstatement.
IA do what they are told by management, including adhering to materiality levels and the amount of work to be done. They might be told to look for fraud, they might simply audit at the same level as the Ext Aud.
Sufficiency:
Materiality of item Judgement/estimates will increase the amount of evidence Standard of Internal control and whether that is being relied upon or substantive procedures are being used.