I have an SVF model. (1521). The Realtek High Definition driver in Windows 10 is version 6.0.1.7564 dated 21/07/2015. The sound levels for me seem to be identical to pre upgrade (Windows 8.1). This driver was found by Microsoft during a fresh install of Windows 10 - not the Sony upgrade procedure.
I can only offer 3 suggestions:
1. Try to get the slightly newer driver. Is your driver number same as mine except for the last 4 digits?
2. Try the Windows 8.1 driver from the Sony UK support page for your laptop. My experience is that W10 has accepted every single 8.1 driver that I have thrown at it. Uninstall your driver. Download the 8.1 driver. Try it.
If you are not used to messing with drivers please back up your existing driver (in fact back up all your drivers) using free software Double Driver before you do anything.
3. If you can't solve it try this. Your device has 4 tick boxes which are called "enhancements". The last one is Equaliser.
Right click on your sound icon in the taskbar. Select sounds. In the playback tab right click on your Realtek HD device and select Properties. Select the Enhancements tab. Tick Equaliser and then click the small box with ... that appears. Edge up the sliders a little bit to taste.
