Complex sentence: Generally speaking, MP3 players are portable, employing internal or replaceable batteries and headphones, although people are increasingly hooking players up to their car and home stereos-sometimes via a wireless connection-thereby turning them into portable jukeboxes.
Split into simpler sentences: Generally speaking, MP3 players are portable, which means they use either internal or replaceable batteries. Typically, you listen to music through headphones, although people are increasingly hooking these players up to their car and home stereos, thereby turning them in to portable jukeboxes.