So here it is! In AU and NZ it's not being scalped as hard as other parts of the world. Still in stock ready for shipping from My Nintendo Store ANZ for AU$109.99 (probably the price tag is the reason!).
The player is built with a chip-on-board controller + 64MB (yes, megabyte!) SPI NOR Flash. That's quite generous compared to other Nintendo toys (FYI Game & Watch Mario comes with a whopping 1MB of SPI Flash). Probably because the controller is too weak to either render music or decode the encoded songs so they have to store all songs in PCM which would take a log of space.
So yes, the plastic cartridge chips does NOT have any circuits – game BGMs are all stored on the player, selected by plastic bumps on cartridge chips pushing 6-pin micro switches, like how a Famicom Disk System anti-piracy works. Counting from left to right when player facing up, there are in total 64 possible slots for 6 bits. With no pin pushed (i.e. no cartridge inserted) plays only the Game Boy chime, and 45 known working combinations from cartridge chips come with the set. That leaves 18 unused combinations. Among all 18 unused, none plays music, except somehow 0b011000 plays Cerulean City Theme, which is the same as 0b101111. Weird.
In case you're interested, here's a full list of music and cartridges:
Pokémon Game Music Collection Game Boy Jukebox – Pin Combinations (Google Docs)
by xmagic5589