You are missing a lot of steps to your story. Which mixer? How is it connected? What are you recording with? How do you generate the MP3? How do you listen to the MP3?
Pro Fx 12 Mixer
Mixer is connected to my laptop = USB used
With Audacity - I opened the program and then hit record
We exported the recording as a MP3
Played on my laptop
Make sure you’ve selected the USB mixer as your Recording Device. My guess is that you’re recording from the mic built-into the laptop…
The USB output from the mixer should be “accurate”.
Also try exporting as WAV before making an MP3. MP3 isn’t terrible (depending on the quality/bitrate) but it is lossy compression and it’s a variable you can eliminate, at least temporarily.
I wouldn’t use that. After plugging in your mixer, in Audacity do a Transport > Rescan Audio Devices, otherwise just restart Audacity. Under Recording Devices in Audio Setup, you should see your mixer (It may be displayed as USB audio codec or somesuch - to be sure, it should go away if you unplug the device, then do a Rescan)
For recording channels, choose 2 (stereo) (or the maximum number available). It doesn’t hurt to to the scratch test (lightly scratch your fingernail across the grill of the microphone to be sure you have the right one).