The other big earner which wasn't mentioned here is publishing income. Now this only works if you're the songwriter but having an enduringly popular back catalogue can be worth a lot of money. David Bowie famously sold bonds on a ten year income that was worth $55 Million.
On a smaller scale, I can't find a source right now, but I remember the NME carrying an article about Lee Mavers who wrote 'There She Goes' and that single song earned him yearly royalties of around $50,000.
So you can imagine, all you need is a small back catalogue of well known and well played songs and the residual income can be pretty good.