Not necessary but I use Audio Hijack in conjunction with Loopback to easily select audio sources I want/don't want as my input audio.
Your input or output device or application names may vary. Below is an animated demonstration to show the essentail blocks you'll need to add to overdrive your audio and start a session. I suggest using 3x or 4x with great care turning system audio very low and then bringing it back up to a comfortable level after starting the session and selecting 3x or 4x to avoid blowing out the drivers on your earphone/speakers etc. Be warned, I have blown out a pair of earphones using this. This has been the only thing I've found that actually makes the volume louder using software rather than just distorting the audio. Select your Output Device (speakers, earphones, etc), and then start the session. Options are: Off (no overdrive/100% volume), 2x, 3x, or 4x. To achieve your goal, Add your sound input(s) (Input Device, or Application) and then add an Overdrive function. You don't have to record if you don't want to. It's a digital recorder with a real-time audio effects engine. Audio Hijack by Rogue Amoeba can legitimately overdrive your audio, among other things.
