A fraction of a volt is audible in headphones.
A microphone signal has amplitude in the area of 10mV, and needs to be amplified.
I suppose 50-100 mV of hiss amplitude is strong enough to hear in headphones.
To reduce hiss, avoid making too much gain (voltage gain) in your amplifier. If you apply excessive gain then you must attenuate your input signal, yet this does not reduce the level of hiss generated in your amplifier. There are popular audio IC's which can be configured to deliver 20 gain or 200. In many cases your headphones mainly need current gain rather than voltage gain since many devices produce signals at line level, nominally 1V.
There are IC's which dynamically reduce noise. Example, LM1894.