Chain creation:

Open audacity

File > Edit Chains

Add new chain, name it what you like. I use "radio voice (highpass8 s 500)" because reasons.

On the chain edit window, you see just one line that says -END-. Insert a new action into the chain.

A dialogue window saying "Select Command" pops up. Don't select anything, just hit OK. This creates an action that applies whatever Nyquist Prompt command was last used in Audacity. This will be important later.

The following actions are less complicated to add. In the order from top to bottom, they should be:

Leveller    dB=-70dB Passes=2                                 (default options)
Normalize   ApplyGain=yes RemoveDcOffset=yes Level=0.000000   (default options)
StereoToMono
ExportWav
-END-

Chain usage:

Drag any file to audacity, or just create a new track with some tone or static on it. It doesn't really matter, as this is just a step to initialize the Nyquist Prompt so that the chain applies the correct command as well.

select Effect -> Nyquist Prompt

run the following command:
(highpass8 s 500)

...and that's it.

What this does is apply a high pass filter with quality 8 (highpass8), applied to a stereo track (s), blocking frequencies below 500 Hz. The other stuff in the chain is for clarity and normalization purposes, obviously.

Once you've applied Nyquist prompt once, you can Apply Chain to either any individual file (apply to current project) or a bunch of files like batch processing (apply to files).