There is no such thing as full volume, and there is not such thing as zero volume.
Ultimately people who use this a lot would choose to become familiar with dB, as they always have. But there's no rush.
Surely constant unchanging air pressure has zero volume?
Volume is the range of variation in air pressure, right? That can surely go down to zero.
It can also only meaningfully go up to about double the absolute mean air pressure, before what you are talking about becomes shockwaves of overpressure.
Volume can be a statistical property, like temperature.
Zero volume is when a speaker's diaphragm is still. On an 8-bit PCM audio file, it corresponds to a value of zero.
In the context of a signal, full volume corresponds to the 127 value in an 8-but PCM audio file (or arguably -128). In the context of a speaker, those values should push the diaphragm no further than how far it can travel linearly without distortion. Obviously the user may want to turn down the volume from this full volume.
I hope you understand that even when using an audio editor that displays values in dB, the underlying values are integers (or floats) that absolutely have zero and "full volume" meanings, and conventionally map respectively to -∞ dB and 0 dB.