A big thanks and tip o' the hat to Malcolm Rayfield for a concise and complete solution. With both volume sliders I can get 12 dB extra gain. On my particular videos in question, their dreadfully inadequate audio levels don't clip & distort when raised, but rather become listenable. The accompanying boosted noise floor is a small price to pay for salvaging my formerly useless videos, and since it is speech, a tweek downward in the treble helps.