You need something like this (https://www.presonus.com/products/HP2) for stereo sound and enough volume. The aux outputs of your mixer are sending line level, which is good for amplifiers, but not for headphones. You will need to link two auxes for a stereo picture.

Another way to listen to your aux mix is via the headphone output and soloing the aux output.

ok i didn't knew that, thanks a lot! and about listetning on the headphone jack, i tried it but i don't know why i get a little little latency, but on the aux mix i have 0 latency. how else can i monitor my recording on headphones? and thanks a lot for the help!!

Inside the mixer audio is processed digitally, so there is no latency at all. AD/DA conversions are identical on all inputs and outputs. Therefore I suppose the latency comes from the recording and how audio arrives at the mixer. Could you tell us more about the equipment you use? Is FireWire involved? How do you record and how do you play back the tracks?

You can use any output for listening to your tracks. The "CR (Control Room) Output" is the output you most probably would use in a studio situation. The aux outputs are normally used for musician's monitoring (wedges, headphones/In Ear monitoring).

thank you for the help, i'm recording with the mixer into reaper with the firewire cable, so i had this simple set up: a mic for recording voice into input 1, recording into reaper and on reaper i turn on the monitoring. i've got the speakers connected to the CR outputs and everything sounded OK, no latency, BUT the singer was on another room and listening through headphones, so i connected them on the aux mix output and i got the problem of only one side getting sound (but i already understand why, thanks) so i proced to connect the headphones on the headphone output and then the singer got a little latency on his mic, but i heared no latency on the control room. why is that?

I suppose the latency comes from the DAW, so I would not use the DAW as "mixer". Why not playing back all recorded tracks through FireWire to your SL 1642AI and do the aux mix/cue mix on the board? Your mixer has all options (EQs, pans, reverb) to make monitoring very comfortable and the DAW/the computer is not occupied by unnecessary processing. You might also want to compare Reaper to Capture. Capture is not a DAW, but it's perfect for recording and playing back tracks. Capture is easy to use and incredibly fast...

Another reason for latency might be a wrong FireWire connection. Your mixer and your computer need a FireWire 800 "bilingual" connection. A 800/400 FireWire connection has a reduced transfer speed, which you might hear as latency.