Between JamKazam and your audio interface, there are many places where you can set levels. It's not hard to mess things up. Typically, users run into trouble when one stage overdrives the next one.
Here's an approach that seems to work well.
1. Create a private session. By the time you get here, you should be able to hear yourself somewhat in JamKazam.
2. Reset your JamKazam levels to the defaults. From the Manage Menu, choose Audio Settings > Configuration > Reset Audio Volumes.
3. Adjust JamKazam Input Levels to 0db. (You can skip this step if you did Step 2) Assuming you have a mic connected to your interface, mouse over the speaker icon in the "Audio Inputs" column of the session page. A meter and level control (fader) appear. Notice that at the top of the meter, there's a "Volume" label, and just below that is a number that corresponds to the level of the fader.
Set the fader to 0db.
4. On your audio interface, adjust the gain or level for the microphone so that one red light on the meter lights when you talk or play fairly loudly.
5. Adjust your headphone level so it sounds good. See Step 5 note below.
6. Adjust other inputs on your interface - In the Inputs column, mouse over the speaker icon for any other inputs (if you are using them - guitars, etc.) and adjust them in a similar manner (with your audio interface gains, until a red light shows occasionally on the meter.
At this point, everything should sound pretty good to you. When others enter a session, you should only need to tweak levels in your personal mix, and probably not very much.
That's it. Happy jamming!
Step 5 note:
A number of availalable interfaces are "bus-powered" - that is, they get their power from your computer's USB cable. For bus-powered interfaces, the power to drive the headphone amplifier is limited by the computer's USB port output.
For some of those interfaces, the impedance of the headphones is critical. If your headphones are the wrong impedance, the headphone amplifier may not be able to get the headphone volume up to a suitable level. (Step 5 is critical to getting the levels right throughout the system)
If you find you have to turn your headphones all the way up in order to hear yourself, your headphones are probably mismatched to the headphone output on your interface.
There are two ways you can remedy this:
1. Try a different pair of headphones. For testing purposes, if you have a suitable adapter, you can try a pair of earbuds. If you can get the headphone level up to a good level without turning the headphone volume all the way up, you have proven that your headphones are mismatched. You can continue to use the earbuds, or look for a different set of headphones.
2. Use a headphone amplifier with the headphone output of the audio interface plugged into it. Headphone amplifiers are pretty much all powered externally, so the output is usually considerably higher than what many bus-powered interfaces put out.