Hello Amelia162
Welcome to Microsoft Community.
If after you press the power button to start the Surface Pro 4 properly, you notice that the volume drops directly to the minimum and automatically lowers even if you manually turn the volume up, then the problem may be related to the audio drive or, as you might suspect, the volume button may be malfunctioning.
After booting your Surface Pro 4 device normally, right-click on the Start menu - Device Manager to find the audio driver in the Sound, Video, and game controllers. Right-click on it, select "Uninstall Device," then click "Scan for hardware changes," and Surface will redeploy the audio driver. Please check to see if this problem still occurs.
If the above process does not fix the problem, please visit the following link:
Download Surface Pro 4 Drivers and Firmware from Official Microsoft Download Center
Download the driver and firmware package for the device model and install it manually. After all this is done, restart your Pro 4 and confirm if the problem will occur again.
Another solution that might work is to try going into Surface UEFI, manually turning off the on-board Aduio option, and then trying it again. This will allow your Pro 4 device to turn the audio device back On. Here are the steps to get into Surface UEFI:
How to use Surface UEFI - Microsoft Support
If you find that the problem persists, then it may indeed be a hardware failure with the volume button.
However, because this model has been discontinued, out of warranty support and service for such models is no longer available. We appreciate your commitment to Surface and apologize for any inconvenience.
We'd love to help you upgrade to a new. Please check out our****online Microsoft Store, visit one of our locations in person, or chat with an expert to learn about ongoing and special promotions.
Best Regards,
Mitchell | Microsoft Community Support Specialist