It all depends on the project and context. For smaller projects that involve new clients, I always require a 25% upfront deposit to begin work. Then I finish all fo the job and get paid at the end. For large(r) projects, I still have the 25% upfront deposit and then talk through milestones with the client. Perhaps split the game up in thirds. Then I invoice as each segment is submitted and approved. 

For clients I've worked with before and trust, I waive the upfront deposit. 

Again, it really depends on the team and project. 

I've had milestones like so: 

1) Establish how many themes will be needed across the entire game. (Pre-production)

2) Get a working version (i.e concept) of X amount of these themes. 

3) Get approval of these concepts. Get paid a percentage.  

4) Create finalized versions of these themes. Depending on your role in the project this could also involve implementing the audio. 

5) Get approval of the finalized versions. 

6) Get paid rest of the balance. 

At any time scope and direction of the game may (and usually does!) change so then you have to reassign the scope. 

I've also had milestones where I literally created some sound effects, threw them over to the client and then got paid while they did everything else. Then I've had projects where I created all of the sound design, music and implemented and set up everything myself. It all depends. There's not set standard. It's literally a by-each-project process. 

Sure!