Step 25: It is complete
The chip I used has a gain of 2 so it needs line levels (around a volt rms) in order to work.
With no signal, the current drawn from the battery was about 2 milliamperes. With the music blasting at a comfortable volume in the earphones shown the current went up to around 12 milliamps.
I should wrap it up in duck tape to make it look nicer, but it looks like the junk it was made from.
I shall try building it into a marker pen body the next time I get the urge to make one.