Hi PaulStewart9,
Thank you for reaching out to Microsoft Community.
Is this the first time that you are trying to use the 3.5mm jack? Or this used to work before then suddenly stops?
How about the built-in microphones of the device, do they work like if you'll use the recorder app, will it be able to record your voice?
Try to open Control Panel > Hardware and Sound > Sound. Under the Recordingtab, select the headset plugged into the 3.5mm jack then click Set default.
If still issue persist, please try these steps below:
1: Run Surface Diagnostic Toolkit and check Windows Update
2: Update drivers and firmware
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Studio 2 then click Download on the next page.
Choose the .msi file that matches your current OS Build and click Next.
After downloading, run and install the .msi then Restart.
3: Run System File Checker Tool in Windows 10
If this answer your question, we highly encourage you to inform our Community by clicking on Yes or marking my post as an answer. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind Regards,
Marrion Y.