sharkbait

It's basically the same thing as tissue v. Kleenex- MP3 player is a generic term for the unit, iPod is a brand of MP3 players.  For inside armor, a iPod Shuffle (no longer made, but you can pick up overstock sometimes for cheap on eBay, Amazon, etc.) or another brand (don't go too cheap, they'll just die and/or not work well) of small unit comparable to the Shuffle would work.  They're small, lightweight, and play audio, which is what you want.  The iPod Touches are also thin, so they would likely fit in most armors (depending on how snug yours fits).  Some guys use their phones (Apple or Android).  

I use an old Shuffle I have and put just TK Chatter on it (available on the 501st forums, I forget where, but it might be linked from here somewhere), so it doesn't accidentally cycle to another track that's my music or something.  My system is a shuffle with a audio cord into an aker, along with a wireless mic plugged into the same aker.  The same set up could be done with any MP3 player in place of the shuffle- they do the same thing.  You can also skip the mic and just have the chatter, if you so desire (or forget to charge your wireless mic).

Sometimes the chatter goes off when I'm talking, but for the most part I either know it's coming up, or I just wait a second for it to finish.  (You can circumvent this with a push to talk set up, but that's complicated and I would forget to push it half the time when I wanted to talk anyway).  

Hope that helps!  Let me know if you need any more info!

Nice! That’s exactly what I needed. Feel way more comfortable now. Thanks. Very helpful. 

You're very welcome!  Glad to help out!