Why no audio line input on iMac Pro and Monterey?

The tech specs for the iMac Pro only said it had a "3.5mm headphone jack." There is no mention of audio input.

If you have (or can get) a set of Apple Earpods (the old style, with a cable & 3.5mm jack) you could test to see if your iMac Pro has any kind of audio input via the headphone jack. If it works with the Earpods mic then a TRRS jack, wired as for the Earpods, may work for analog, mic-level audio input.

I suspect, however, that your technique of plugging the 2-ring cable only half-way into the iMac Pro's headphone jack is just producing a false indication of an "External microphone."

imho a USB audio interface is a far better way to get analog audio input to a Mac that does not have a dedicated line-in port.

What says the headphone jack also has a mic connection?

Older iPhone and similar devices uses a 4-conductor plug. I find nothing that says the headphone jack on iMac Prod has a mic connector

tHe description for the USB-C>3.5mm receptacle says it is for audio like headphones or spears, nothing about mic.

You may need something like this

USB Type C to 3.5mm Female Headphone Jack Adapter, JSAUX USB C to Aux Audio Dongle Cable Cord Compatible with Pixel 4 3 2 XL, Samsung Galaxy S21 S20 Ultra S20+ Note 20 10 S10 S9 Plus iPad Pro(Grey)

BTW, all of the following are good USB audio interface units. Models in each line have different numbers of inputs and other features:

• Focusrite Scarlett
• Mackie Onyx Producer
• Tascam US-2x2
• Presonus AudioBox or Studio

In my own case, I went for a Mackie ProFXv3 mixer w/USB interface. (As a mixer, it has multiple mic channels, balanced & unbalanced inputs, line level inputs, EQ, effects, stereo & mono inputs and more.) It fit my needs much better than the standalone interface units.

(Note, This is based on my personal experience only. I do not receive any compensation for suggesting any products.)

Sorry you're having so much trouble with this issue. Check Audio-Midi Setup (in the Utilities folder) to see if it recognizes this adapter as an Input. If so, then make sure you select a format that matches what the Sabrent device supports.

No guarantees this will work, the Sabrent and similar inexpensive audio adapters do not have a good reputation for working.

i'm not experiencing any issues with the USB audio device i use. have you gone into your Audio MIDI Setup and ensure that a) the input volume isn't turned all the way down? and/or b) the channels are not muted?

Scenario wrote:

There is no UI in Monterey's Sound Preferences to tell the headphone jack to switch to input mode, as in previous versions of macOS.

FWIW, the last Macs with Audio In were in 2011.