I had same sound card volume issue with Surface Book 3. I found that, at least for me, it is was due to the selection of different version of sound card driver. It seems that the operating system could not automatically choose the latest driver even though the driver has been downloaded and could be seen from Device Manager. Therefore, it is a rather straightforward fix for me:
Step 0: Get your Surface Book 3 updated by "Windows Update" inside "Settings" window.
Step 1: Right-click Windows button at the low-left corner of the desktop and go into "Device Manager".
Step 2: Go to "Sound, video and game controllers" and right-click "Realtek High Definition Audio(SST)" and choose "Update Driver".
Step 3: Inside "Update Drivers" window choose "Browse my computer for drivers".
Step 4: Choose "Let me pick from a list of available drivers on my computer" and click "Next".
Step 5: Choose the latest version of available driver for your Realtek audio device and click "Next". . (as of June 2021 I have driver ver. 6.0.8936 installed on my Surface Book 3)
Step 6: If the driver was updated successfully(the sound will be muted for a moment), it would ask you to restart your computer before the latest driver could be used. For me, the above steps got my Surface Book 3's issue of "sound volume too low" solved.