I've been doing some work lately on some faulty sound boards and old drum machines, and found it quite tricky to work out exactly what part of a circuit was faulty. You can't really use a logic probe to help, and a 'scope can give difficult to understand readings.
I had a look around for an audio probe (which is used to probe parts of a circuit and hear what's going on), but couldn't find one at a reasonable price, so armed with the LM386 amp datasheet I set out to build my own. A PCB was designed, plus around £7 of components later and I've a working audio probe.
The probe connects to the power supply of the circuit under test (usually the 0V & +12V of the sound section), you just need to plug in a pair of headphones as it doesn't have a built-in speaker. The sensitivity and volume are adjustable, and you just touch the probe tip to the part of the circuit you want to "listen to".
It helps answer the "is the power amp or some other part of the sound circuit faulty" predicament as it can prove the working bits of a circuit to narrow down to where a fault actually is.
Photos of the one built unit are attached (the probe tip is a nail, until the proper tips I've ordered arrive from China!)
Bare PCBs are available if anyone is interested, £2 each plus postage