Reducing power consumption of an arduino module (mosfet?)
The first circuit will not work, you need a P-Channel MOSFET to switch the top rail with a pull-up. Note the logic inversion here, you pull the line low to turn on the power.







simulate this circuit &ndash; Schematic created using CircuitLab

You need to add some bulk capacitance to compensate for the diode which will introduce some interesting noise on the power line.

The gate resistor needs to be small, the pulldown/pullup needs to be large as shown in the schematic above.


  In low-side configuration, do I still need the TX/RX resistors?


Yes. The resistors prevent the signals from trying to power the device. 1K may actually be small. When you park the player you should also "park" those data lines in whichever state doesn't pass current to it.


  What are some good options of widely available mosfets? IRL540?


Advising on devices is beyond the scope here, but you need a device with a gate threshold under 2.5V here and on on-resistance in the 10s of milliohms range. If the player is drawing 1A that resistance, and power lost in the MOSFET adds up.


  Should I add a ceramic capacitor to eliminate any noise from speakers/sudden change in current, and what sort of value?


Now that is a whole other can of worms. It depends on how the module itself handles power on. You may find it internally sequences power up properly, or you may discover the speaker "pops" every time you turn the module on and off. If the latter is the case you may need to add and control another device to disconnect the speaker while you toggle the power to the player.