Basically any electroplating tank uses <5V, no matter how large it is. So are basically all line-level audio and most dynamic speaker drive signals: 5V at 4Ω is 6 watts, which is a fairly loud speaker.
Detecting 1-volt voltage drops is not at all difficult; that's enough to turn on any BJT, and any random opamp can measure voltage differences down to millivolts, often nanovolts. https://www.diodes.com/assets/Datasheets/AS321.pdf is a 50¢ random opamp, the one Digi-Key has the most in stock of at the moment; its offset voltage is specified as 5 millivolts max, but of course it can measure much smaller voltages than that if you null out the offset with a trimpot, or if you just don't care about it.
This is not Arduino's strong point, but it doesn't have any difficulty with that task either. The ADC in the ATMega328P used in most Arduinos has a resolution of about a millivolt when referenced to its internal bandgap https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-..., and it also has an analog comparator with a maximum offset voltage specified as 40 millivolts. And any random cheap-shit multimeter can measure down to a millivolt or so. And, from the discussion above about audio line levels, it should be obvious that just about any dynamic speaker, and most headphones, can easily make millivolt-level signals audible.
Maybe you meant "detecting voltage drops in analog of <1 microvolt can be challenging".