I designed the audio transcoding infrastructure and scaling plan for an audio-heavy startup a number of years ago, but unfortunately the mobile app was delayed and they ran out of money before public launch.

Using other servers as lower priority job servers is a great way to increase utilization during off-peak times and save on server costs. You have to think about what happens if either a batch job or the server's main task runs away with all the system resources, so it can get tricky. A spot market might actually be an interesting solution to some of those problems, though for most of us, just using disk quotas and nice levels will be enough.