Foxhole radio not working
I don't know your speaker, but you have to realize this kind of receiver has a very high output impedance: there's nearly no current this can supply to the speaker.
Also, you won't be able to see audio-frequency AC voltage with your voltmeter.
You'll need an earpiece (usually, not something you'd call a &quot;speaker&quot;) that has a very high impedance.
Think about this: that's a passive circuit, which means all the energy reaching your inner ear has to come from the electromagnetic wave you're picking up with your antenna. That's microwatts, at best.
Your audio amplifier driving a speaker (e.g. in your stereo, or in your smart phone) have powers of 0.1 W to 100 W, depending on how large and loud you want your speakers to be. So, you're off by at least 5 orders of magnitude.
And that assumes your circuit works perfectly well (I don't even think yours really does).
Also, these razorblade diodes are far from deterministic: it's usual that you'd have to spend minutes until you find a spot where it works.