You will need to provide your own IN-the-EAR-CANAL, ISOLATING earphones. There are many available but I recommend the KOSS "SPARKPLUG", they have 16 Ohms impedance and at $15 are a good trade-off between price and performance. You can pay a lot more money for a set of earphones and if you do, the Honker HEARAMP will let you get your money's worth out of them. The amplifiers used in the HEARAMP are high-quality, low-distortion components as good as any CD or HI-FI stereo. Higher-Ohms earphones will generally allow more audio power. (Peak power between 25 - 30 Ohms.) You won't need it, but that's what happens.
The HEARAMP has its own on/off switch and volume control. This lets you save battery life if you don't need to use the monitor. With the HEARAMP on, the BLUE LED will glow. When the battery starts to get low, this LED will flash. The battery monitor is very sensitive. It may flash when you are blowing and then stay steady on when you stop. When it flashes while it's quiet, it means you have about 6 hours of battery left. (With an Alkaline battery)
The flashes will get shorter, that is, it will stay off longer between flashes and each flash will be shorter as the battery goes down. When the flashes quit altogether, you will still have at least an hour of battery life left. After that, you will start to hear distortion in the earphones. This will become VERY noticeable TO YOU long before it will make any difference in the Honker output. All these times are based on an Alkaline battery. With the Lithium battery it comes with, you can multiply the hours by 4.
If you need to use a MONAURAL earphone (Remember, BOTH ears should be plugged with SOMETHING.) or if you want something else playing in the other ear, you will need to remove the JUMPER, inside the box. Take the back off, (take the 4 screws out with a #1 Phillips) pull the jumper off the two pins and plug it back on the one closest to the output jack. When you put the screws back, tighten the short ones until just snug. (Don't overdo it on the long ones either.)
The amplifier has circuitry to reduce the pop when it is turned on or off but you should still be careful when connecting or unplugging things with the earphones in your ears.