You didn't mention what you are using to record, but my guess is that the passive mixer is a mistake. Typically the passive mixer is for taking multiple line level inputs and combining them to a single source. Some nominal levels which are used in recording include:
+4 dbv - professional audio nominal level
-10 dbv - consumer audio nominal level
-30 dbv - musical instrument output (ie: electric guitar/bass)
-45 dbv - condenser microphone
-55 dbv dynamic microphone
If the interface came built into the computer, it is likely that it is running at consumer level. Still there is a need to amplify musical instruments or microphones for best sound quality.
If you are running microphones into a passive mixer (which creates an even greater loss) when they need 40 db of amplification, the signal will sound weak.
If you are recording a single channel at a time, a microphone preamp will do. If you are recording multiple channels at once, any quality mixer will be an improvement. If we know more about what you are doing and your budget, we may be able to recommend some equipment that may help.
Galil