[a fool in paradise]

When did the basic idea become practical? The introduction of the Sony Walkman in 1979 kicked off the era of pervasive, portable prerecorded music.

I'd point to the transitor radio.

And when Sony first tried to make pocket sized electronics, they made shirts as well with a slightly larger pocket.

[ShadowAce]

That’s a way of looking at it, though I’d tend to align myself with the Walkman here. Radio was still radio—just smaller. Once you could get the electronics for tape playback, then things started picking up.

[Calvinist_Dark_Lord]

And when Sony first tried to make pocket sized electronics, they made shirts as well with a slightly larger pocket.

My late mother left a Sony transistor radio from 1962 among her effects.

It used a 4.5 V battery about the size of a present AA cell. It is smaller than a Sony Walkman, with earbud. Still works, AM only.