Connecting the left and right direct can be a bit iffy, but there are alternatives. You could insert a resistor, value not really critical, but I would go for 47ohm each or greater into each of the signal legs, and then common the pair. Optionally, this common point can be connected to the -ve line via another resistor of the same value. The resistors are about 13P each.
This will effectively stop the left and right MP3 player amps from tryng to feed each others outputs. The combined outputs are now ready to connect to a mono input.
The MP3 player will do all of the preamplifying you need.
The N64AW is a magic brick. Mine has had all manner of abuse, including being well dunked in salt water, and still works.
Of course, if you have two MP3 players, you could get one sound from the left of one and the right of the other..........