When decoding a digital signal with an RTL-SDR the digital audio is usually piped from receiver software like SDR# via stereo mix or software like Virtual Audio Cable/VBCable into the decoding software. The decoding software expects a clean audio signal with the volume levels set not too loud, but also not too quiet. Usually this can be achieved by trial and error.
However amateur radio hobbyist K3RRR found that setting the audio level correctly was critical for maximizing decodes on his digital HF signals when using his RTL-SDR. To correctly set the audio levels he uses a program called Peak Level Meter and another called Soundcard Oscilloscope. Peak Level Meter is used to ensure that the audio levels are set correctly and Soundcard Oscilloscope is used to ensure that the audio is not being over driven into square waves.