Professional singer Ambroz Bajec-Lapajne sang opera during neurosurgery for a brain tumor, at his physicians' request so they could monitor his singing ability and "avoid deficits after the procedure," he writes.
Keeping patients awake during brain surgery is common to reduce risk of complications or damage later. From Bajec-Lapajne's YouTube post:
I sing two (first and last) couplets of Schubert's lied "Gute Nacht": the minor – major transition in order to see if I can still recognise the key change. All is fine until min. 2:40 when things start to get very interesting… It's been more than a year since and I'm doing fine, continuing my professional singing career….
In a comment, he added:
I'm just a singer and tenor at that… I believe he rewired my brain for a while and that was the result. I could not control my tongue anymore and could not stop phonating. It was a very weird feeling.
And in case you missed it, earlier this year a fellow played Beatles tunes on his guitar while undergoing brain surgery for a tumor. Video below and more info here.