Internal audit metrics
I think there's one, and only one, valid metric for internal (first party) audits; findings by 2nd & 3rd party audits! I don't say "# of findings" 'cause very often external audit n/c's are "bogus." (To quote Mr. Jeff Spicolli, senior emeritus, Ridgemont High.) So, look at the validity of the 2/3 party findings.
So what if you're done on time, on schedule, on budget, and the external audits are digging stuff up? It isn't doing its job!