I'll advise you to not build this yourself, and instead spend the money on something designed for this task. If you value your hearing as much as I value mine, it is money well spent.
Simply taking the signal from a mic and amplifying it and sending it to a headphone is relatively easy. On a scale of 1 to 10, it is a 1 or 2. Doing it with high audio quality and low noise is about a 2 or 3. Doing it with safety is about a 5 or 6. I would argue that anything 2 or beyond is not something a beginner EE is able to do as their first or second project.
So I've mentioned safety a couple of times, so here is what I mean: The amount of amplification required for this is huge. Not unreasonably huge, but still huge. Let's say that you're at a safe with this device in your ear and your cell phone rings, or something falls on the safe, or someone is behind you and talks loudly. That sound will be amplified a huge amount and sent into your ears. Without some safety circuits, you will receive ear damage. Maybe temporary, or maybe permanent, but still damage.
A proper device will have something called a "limiter", which will detect the loud noise and reduce the volume before it gets into your ear. It turns out that designing a limiter that both sounds good and protects your ears is a difficult thing. You can roughly divide up the limiter types into two categories: Analog and Digital.
One component of a good limiter is an audio delay. A short delay (less than 100 mS) is required to properly detect the audio peak and reduce the level BEFORE it goes to your ear.
If you look up limiter circuits on the 'Net you will likely find analog limiters. The problem with analog limiters is that it is hard to do an analog audio delay. It can be done, but it is difficult and often sounds terrible. So almost all analog limiters do not have a audio delay. This is often OK for limiters for recording music, but is NOT ok for protecting your hearing. The part of the circuit that turns the volume down does not respond fast enough to protect your ear (the audio delay compensates for this).
Digital limiter circuits get around this by doing things digitally. Normally this is a small DSP plus ADC and DAC (sometimes all integrated into the same chip). Then some software is written for the DSP to do the limiting function.
For an application like this, the DSP would be used for other things too. It might enhance some frequencies while removing others-- all to make safe cracking easier.
For someone who doesn't know electrical engineering at all, and probably doesn't know the math behind digital signal processing, this project is probably way beyond your ability. This isn't an insult, just reality. We can't all be proficient at everything. You could learn it and become proficient, but that can take a long time. Maybe years.
Disclaimer: It might be possible to make an analog limiter that does protect your hearing and doesn't require years of training to do. But I can guarantee that it'll sound terrible and not be near as good as if it was done correctly. I also would not want to be the guy who is the beta tester.
