Here are step-by-step instructions on how to configure audio for good decoding results.
Do NOT plug the audio cable in just yet.
Run Uniscope in Oscilloscope mode; select the input you think is right, use 16 bit samples at a rate of 44100, 48000, or 96000 samples per second. Higher is better but not all hardware support higher rates (most modern stuff does).
Don't use the "Default Sound Input". Some audio drivers can detect that you've plugged something into a different sound input and will switch the Windows default input accordingly. This is bad as it may break your configuration after you've gotten everything working.
Is the 'scope trace flat?
No: you've selected the wrong input (or something is seriously wrong) - select a different input for Uniscope and start over.
Yes: great, let's move on.
Plug the audio cable into the sound input (which ever line-in or microphone input you think is correct). Observe the 'scope display while doing so.
Did the 'scope trace jump as the connection was made?
No: you've selected the wrong input - plug the cable into a different input or go to the start of the these instructions, select a different input for Uniscope and start over.
Yes: Great, let's move on.
Our goal in this next section is to adjust for a good signal level. Too low will produce anemic waveforms that aren't decodeable. Too high will clip the positive and negative extents of the waveform resulting in a loss of information. You'll need to find the recording level setting for this input. Hidden in the audio recording control panel are the controls to set recording level. Some audio drivers provide a sliding volume control or an on/off +20 db gain checkbox. Others may provide both or neither. After you've located this control - have it ready to make your level adjustment.
You'll want a realistic signal so tune your radio to a known control channel.
Do you see a flat-line or near-flat waveform? (even noise should produce some movement)
Yes: adjust the signal higher (increase level or check the +20db/+30db) checkbox). If the peaks start to flatten - you've gone too far. You'll want to back down just a bit.
Do you see square(ish) waves with abrupt corners?
Yes: too much gain! Turn OFF the +20db/+30db checkbox or turn down the level - or both.
Somewhere between the two is a happy medium that does not mute or clip the waveform.