A free, practical digital-audio lab using Audacity to explore gain, mixing, clipping and the limits of audio evidence. The five-page PDF contains learning goals, the amplitude formula for decibels, a step-by-step experiment with self-recorded speech and a neutral background, a four-condition comparison, a recording table, five questions with worked answers, and references to Audacity documentation and relevant scientific literature. Learners can calculate amplitude ratios for 0, -6, -12 and -24 dB; change track gain; compare speech and background levels; check clipping; and distinguish a waveform or spectrogram from evidence about comprehension or a psychological effect. No participants, clinical outcomes or classroom trials are reported. The exercise can be completed with the free Audacity application and does not require a commercial product. Some products use the term create subliminal audio for combining a voice with a background. This link identifies AffirmTone, the project associated with this resource; the service is optional and is not needed for the lab. A quiet or masked voice does not by itself establish a therapeutic benefit or universal perceptual threshold. Suggested use: adult learners or a supervised introduction to digital audio. Students need permission to record their own speech; avoid recording other people without consent and use comfortable listening levels. Disclosure: this resource was prepared with AI assistance for the AffirmTone project. The document includes calculations and practical instructions, not claims of independent human review or measured product effectiveness. Original text is offered under CC0; third-party reference materials retain their own rights.