I have two digital audio sources that should not be on at the same time as each other. One gives out a 5v square wave, the other a 3v square wave. Will I be OK to diode mix these into the amplifier (a logic level FET)?
1) You do not mix digital sources.
2) A logic-level FET probably will not work with a 3.3V signal that is reduced to 2.6V with a diode. It needs at least 4.5V.
Sorry - I should not really have called it mixing. It is simply that one is on while the other is off, and I don't want to blow up the output of the other.
I'm using a LDO schottky diode, and I've found that my FET does turn on at the 2.9v there is on the gate. It is a small speaker, so even thought the Rds of the FET is ~1.5 ohms, that's no problem in this case.
You are feeding DC pulses to a speaker?
But a speaker uses AC, not DC.
An audio amplifier has a very low output impedance (0.04 ohms or less) that damps the resonance of a speaker. Your FET has 1.5 ohms in one direction and infinity (boooom) in the other direction. The speaker will sound like a bongo drum.