Let's Build some PC Armies

[MaxWilson]

Both. If you observe the suspect, you'll notice any 'clipping issues', you'll notice if the sound doesn't come at the right time, and you'll notice if someone's casting Minor Illusion to fake the sound. You can also tell them to hit the shield in specific pattern, so MI sound can't be prepared in advance.

MI sound doesn't need to be prepared in advance, and doesn't require an action to control.

More significantly though, if you're close enough to detect clipping issues you're already too close--you've given up the range advantage which you otherwise would be able to rely on. So this is good only for preventing small-scale, long-term infiltration, which Disguise Self isn't good at achieving anyway. It won't prevent short-term Trojan Horse maneuvers designed to get close.

I don't think you'd even need to go that far, the spell explicitly calls out that it fails to hold up to physical inspection that sounds like an auto fail whether or not the can replicate the sound.

I think you're talking about something different than we are.