Here's the manual:
A volume of 0 dB is original volume, and below 0 dB attenuates the sound. Note amplification is not supported. For example, entering a value of -10 plays the audio back 10 dB quieter (about half as loud). Prefer setting the intended volume in the Play action; even if followed immediately by a Set volume action, some platforms will momentarily play the audio at the volume given in the Play action.