line in/aux input on 2017 MacBook Pro(MBP)...new to MAC user.....Garageband Forum

Solution: There is an adapter made by Sabrent. USB-C Model no. AU-MMSC

It costs less than $10 to buy. This one is for my 2016-MacBook Pro.

I also use the USB 3.0 one with my iMac Pro. It works GREAT!

The adapters are sold at electronics stores. I bought mine from Frys.

I am currently running Mojave 10.14.5 on the latest release iMac Pro.

I am experiencing the same exact issue you are having. Mine is an iMac Pro. It only has a single headphone jack. In previous years the iMacs had a separate headphone and mic jack for 3.5mm plugins.

I have even done extensive research for a solution and bought a couple of adapters seeking resolution. Unfortunately, to no avail.

The two adapters I purchased were, USB-C to 3.5mm headphone jack(female) from the Apple store. And a 3.5 TRRS male tip to split lines(female), which are 3.5mm mic input and 3.5mm headphone input from the Amazon online store. When I use my headphones with the built-in mic, the iMac Pro detects both audio input/output with no issues. Whether I am using the Apple store adapter or not.

However, when I use the adapter for any other input that isn't headphones, (mic, auxiliary inputs from mixer) the devices or mic is not detected.

Also, the Audio MIDI Setup.app is useless because the iMac Pro does not detect a mic plugged in at all in these instances.

This is really disappointing.

Does this app allow you to make the needed changes—

Applications/Utilities/ Audio MIDI Setup.app