Pub 463(2015): https://www.irs.gov/publications/p463/ch06.html
Per diem or mileage allowance exceeds the federal rate: Adequate accounting up to the federal rate only and excess not returned. The excess amount as wages in box 1. The amount up to the federal rate is reported only under code L in box 12 of Form W-2 — it is not reported in box 1. All expenses (and reimbursements reported under code L in box 12 of Form W-2) only if expenses in excess of the federal rate are claimed. Otherwise, form is not filed.
That's for employees, I think the excess would be the same for volunteers.
Example 3.
Debbie drives 10,000 miles in 2015 for business. Under her employer's accountable plan, she gets reimbursed 60 cents a mile, which is more than the standard mileage rate. Her total reimbursement is $6,000.
Debbie's employer must include the reimbursement amount up to the standard mileage rate, $5,750 (10,000 × 57.5 cents), under code L in box 12 of her Form W-2. That amount is not taxable. Her employer must also include $250 ($6,000 − $5,750) in box 1 of her Form W-2. This is the reimbursement that is more than the standard mileage rate.
If Debbie's expenses are equal to or less than the standard mileage rate, she would not complete Form 2106. If her expenses are more than the standard mileage rate, she would complete Form 2106 and report her total expenses and reimbursement (shown under code L in box 12 of her Form W-2). She would then claim the excess expenses as an itemized deduction.

