The IRS is nothing if not exhaustive (and exhausting). It wants to know about every red cent you earned during the tax year, whether it was through a full-time office job, a home-based cupcake business or part-time waitressing gig for tips. If you are paid by direct deposit, check, cash or rolls of pennies, it all counts as earned income.
The IRS receives copies of all W-2s and 1099s sent by your employer or clients, so there's no getting around that income. But don't be fooled into thinking that cash income, payments from foreign companies, and other types of income that don't generate a W-2 or 1099 are somehow immune to taxation.
This is the "voluntary" part again. It's up to the taxpayer to honestly and accurately declare all earned income. If you are audited, the IRS is going to ask for copies of your bank statements. If you can't explain where all of these mystery deposits came from, you will be in deep doo-doo.