teh datassette is mechanical and how would it pause if the commodore orders it to?
There's some electronic parts in a C2N as well. The connector C/3 on the cassette port (that's the second from the left when viewed from the rear of the C64) can control the motor of the datasette. If there's +5V provided on that port, the motor will start IF the mechanic allows it (i.e., if either play, forward, rewind or record/play is pressed). 0V will stop the motor. Yes, that means if you fry that connector the datasette motor won't spin up at all even if you press play on tape. ;-)
BTW, you can control the motor using some Basic! Just change the addresses 1 and 192 like this:
POKE 192,0:POKE1,PEEK(1)AND223 - turn the motor on
POKE 192,1:POKE1,PEEK(1)OR32 - turn it off.
You can even check if any key on the datasette is currently pressed using WAIT 1,16,16
I checked on that subject and found that the header part uses a synchronization that's different from the program part, plus there's a brief pause between the two parts. The header data is stored in memory. That means, either if you hit the Space Bar fast enough (1-2 seconds will do) OR if you rewind the MP3 and play it again the C64 will automatically recognize the program part and just load it.