MP3 Player: Disconnect 3-watt speaker from amplifier when headphones are plugged in using MOSFETS and the isolated switch on an SMD 3.5mm stereo jack
A single FET won't do the job properly because the signal is AC, and the FET has a body diode that conducts when voltage on the Drain is negative with respect to the Source. To fix this you need two FETs, connected back-to-back.

Another problem is that to keep the FET fully turned on the Gate-Source voltage must be higher than its threshold voltage. Since the amplifier output swings (almost) from rail to rail, the Gate drive voltage must be a few volts higher than Vcc. If your circuit is powered from a higher voltage that is regulated down to 3.3~5V then you may be able to tap into the unregulated supply, otherwise you might need even more parts to generate the required Gate voltage.      

Assuming you have a sufficiently high voltage available, the circuit could look something like this:-