I picked these up more as a novelty. This is one of the better-made cheap players- good sound quality and aluminium body, but the purple one was slightly damaged due to a bend in the metal case squeezing on the inside frame, so I took that one apart. This model lacks screws, but a locking button on top can be pushed down to slide the guts out.
These are basically clones of the 2nd gen iPod Shuffle- but the components and even casing are far simpler than Apple's hardware.
I'd wager a lot of these 5-button players are similar inside. The front is just a PCB with traces and 5 small membrane switches, with a plastic button piece on top. In this case, a square ring for 4 and a separate button in the center for play. They are labeled K1-K5, and the text in the corner reads "AC1187/PGJZ-V12/(BAT)/2015-04-07" the last I take to mean a date of manufacture.
The rear is just a micro SD slot, headphone jack, mini-b USB slot(power and data- the player acts as a card reader too), on-off switch and a single control chip, plus some tiny components including a power diode. The chip reads "AC1187". The Li-Ion battery(edit- it's LiPo, not Li-Ion) is small, no markings and has a heart stamp with "LR" in it. I wonder if the battery could be replaced with a higher capacity type without affecting the electronics? Theoretically, anyway.
These tiny players are simple to take apart and use the insides for projects as it's trivial to rewire buttons and jacks to fit them inside various casings.