You earn gems for simulating games . . . same kind of gems as when you complete a level…
As for “how” has already been answered…
It has a chance to update every so many interactions, like playing levels, etc. Clicking the speaker control is just the fastest way to generate these interactions…
The multiplayer “ladder games” are the ones where you compete against other players, such as: Cavern Survival (dungeon), Dueling Grounds (forest), Multiplayer Treasure Grove (forest), etc. There is currently at least one in every world-region (Dungeon through Mountain). You get to the “ladder” page, usually, by clicking on the level on the map and selecting “Scores”. You only need to simulate in one so pick one and go for it. (Games simulated doesn’t care which level you are on (for “count of simulations” or “level to simulate”.)
If you want to know how many gems, well Search!!, um, I mean, check out this post… 