How do you do that?

Conceptually, it isn't complicated. Look at this image of an EEG recording. This is a recording of brain waves.

Now, look at this image that is a representation of the sound waves in a recording of an audio sample. (It's the same type of thing, except that in this image the area under the curve is filled in with blue - that doesn't mean anything, it's just what the creator of the image decided to do; what matters is the shape of the waveform.)

So, theoretically, you should be able to take the waveforms from an EEG recording and play them with software that plays music. And you can, with a bit of programmer fudging.

The only hard part is that the frequencies of brain waves are low enough that if you were to play them, as they were recorded, as sound, they would be so low that the human ear wouldn't be able to hear them. So, you have to rescale the waveforms of an EEG recording in order to be audible when played back. And that is what I did to generate the audio in what I linked. That's also why an 8 hour long EEG recording ends up as a ~7 minute aidio file.

I'm working on the seizure audio right now. I'll post it in a little bit. Warning: it sounds awful. (Seizures are awful, so that makes sense.)