The energy dissipates as heat
The energy of the brain is electrochemical within a complex system with electrical potential in the range of millivolts. Far, far too weak to do much beyond trigger other, electrochemical reactions, let alone create sound or light. Like a computer, it's a complex labyrinth of signals chasing each other so much, it mostly generates heat, and it doesn't do any useful work. Rather, it controls other organs that do useful work. You'd get better effects from the death of muscles but that's also too weak to generate detectable light or sound. Instead, the electrochemical potential just gets wasted as heat and the worms and bacteria consume the rest of chemical potential.
If you think the clever synergy of the brain must do something, you watch too much Star Trek. The effectiveness of synergy is tied up in the system that supports it. The state and synergy of a car engine cannot, even theoretically, be copied to a jet engine. They're sets of parts with different systems of interaction. A jet engine has no direct translation of piston position and velocity. The clever energy of the brain means sod all without the neural net it sits in. The electrical state of a PC just doesn't go into a Mac, let alone other electrical devices, let alone anything else.
Of course, a car engine can input energy into another system but it must be transferred in particular ways that the receiving system can inherently accept. A car engine can directly drive a dynamo but not a radio. An engine can indirectly drive a radio but every single intermediate stage must directly compatible with the next. Ghosts from brain energy has the problem of how synapse electrochemistry can convert into anything else besides waste heat. And it still has to get out the head. It could drive muscles in its death throes but nothing sophisticated will come from that. It could escape via lots of electrodes shoved in the brain but that doesn't happen.
In short, ghosts from brain energy must be complex systems that're compatible with human neural nets, can get it out of the skull naturally, amplify the extremely weak signals and be sustained by energy not yet detectable.
Misapplied, pre-historic brain functions, collective mythologies and false memories are starting to look like a simpler explanation.
