This is so much thinking about so little data to make any claim of this one way or the other....
Every cycle of a Charge / Discharge will have less time with usable voltage output than the cycle before it.
You are focused only on capacity or mAh and aren't even mentioning probably the most important part is that the cell maintains >1.2 volts. Most rechargeable cells are barely over 1.3v after a full charge.
The batteries might have nothing wrong with them either, maybe you are moving around too much and the player is forced to re-read more often.
Maybe you're skipping around different tracks too often, causing the player to overuse its motors to fast forward, seek, rewind.
Maybe it's a hotter ambient temperature which will decrease overall efficiency of the cells.
There's just so many variables and you have only used what, 4 different cells of varying ages and mileage put on them? These cells are mass produced too... like you'd have to calculate a margin for error based on the variance of the cells performance straight out of their manufacturing.......
Chalk it up to the cells are bad, get some nice quality high mAh cells and move on!
Cheers.
