I've unlocked most of the 23 available songs in the media player by hacking people passing on the street. I have 20 songs unlocked and there seems to be no more songs unlockable by hacking. I understand that using the SongSneak app near a speaker that's playing music will unlock the song that's playing, but I don't really want to wander into every shop in the game in hopes that I'll stumble upon a song I don't have. Are there certain locations that I can go to where these last few songs are always playing, or is it completely random?
For me, the last few are in places, like bars, poker areas, cafes etc. Walk in, run the SongSneak app, and if you're lucky you'll get one of the ones your missing.
For me, the last one took me 15 minutes of waiting in a Poker room for the song to come on.
See here for a list of all the songs and where you can find them.