After plugging in an unrelated USB device my built-in realtek sound card suddenly decided to go into low volume mode, where the output level was several levels below what it used to be. The mixer in Windows 7 shows the full volume for the application playing (Spotify in this case), but the volume output on the speakers was very low.
Go into the “Realtek HD Audio Manager” on the control panel, select “Speakers” and then “Default Format“. This had suddenly been set to 24 bit, switching it back to 16-bit 192KHz made the volume normal again. Unless you require another format on the digital output, 16-bit 192KHz or 48KHz should be more than enough (I’m guessing this adjust the quality of the internal DAC on the sound card, so go with 192KHz).
Update: turns out this still happens, but changing it to something else, then back to the previous value solves the issue. No idea why.