I do audio processing on Debian Linux, and find that a virtual machine with only one core encodes audio as OGG or AAC just about as fast with eight CPU cores.
Since I lack systematic computer science education, I missed the part about allotting CPU resources, and what I’ve tried so far in Linux seems to be a dead end, so I’ll spare you.
In short, is there a way I can launch an encoder, like fdkaac (non-free) or oggenc so that it will use as much CPU as the system has available?
In order to benefit from several CPU cores, the encoder program you're using should be written in a special way (with threads). This is not the case for oggenc. You can try to use encoders which support threads, like mencoder. mencoder has a threads parameter which you should set to 8 (its default value is 1, meaning no threads).
Another possibility (which is often preferred) is to encode several files at the same time with several instances of encoder running in parallel. In this case, no threads support is required from the encoder itself, as the OS will take care of scheduling encoder processes to available CPU cores.
nice. Levels lower (meaning higher priority) than 0 (usually the default) will require root privileges or a specific configuration.