Game development adds another dimension. A musician I know helps small indie artists with music as a side business. His contract states a fixed number of hours producing the music and effects is free, until earning some 100k revenue / month, when it starts costing 0.1% of revenue royalties / month. He has fun with it, it's usually free, but if someone makes the new Minecraft with his music, he will get his share.
Sure you can. At 100k / month revenue, you have 100 currency / month or 1200 / year available for another artist, which may or may not include the risk of messing up the "feel of the SFX and music". Even at 10x that, that's a paltry sum, especially if identity is tied to it.
