It should be possible. Make sure the Monitor Switch on the UA-25 is on. It should have a separate volume control next to it, and the red light should be lit.
If you have that part down already, it could be the input is too quiet to be heard. You may have to adjust the volumes for the input or monitor control, and possibly the volume within the program or on the unit. Example being -> set the input for your voice higher (without clipping of course), and/or set the monitor volume higher while turning down the master output on
the UA-25.