I used to volunteer to be bumped all the time...I don't know if you can still do it, but I used to sell the bump coupons for cash if I didn't have any plans for travel before they would expire.
I got a voucher when I was bumped. I signed the thing before I read up on it: Signing the thing means you immediately waive your right to cash.
Reading up on it though, including at the Department of Transportation, I could have asked for cash or check.
However, "If your ticket does not show a fare (for example, a frequent-flyer award ticket or a ticket issued by a consolidator), your denied boarding compensation is based on the lowest cash, check or credit card payment charged for a ticket in the same class of service (e.g., coach, first class) on that flight."
I also hear this only rarely: They make an announcement asking for people to voluntarily stay off the flight, and they say that the person will get a voucher for some dollar amount.
Most announcements I've heard leave out that bit of information. They might get more volunteers if they stated that one important sentence.