. I also Experienced similar Volume fluctuation issues, it took me a lone while figuring out this problem
. I found out that It's the Audio drivers
one installs that cause bass and volume problems.
What I did was, I uninstall my Audio Drivers (IDT High Definition Audio drivers) and Just used the drivers Windows 7 automatically installs (When new hardware is detected), and I just used the "Bass boost" and "Loudness Correction" in the "Sound Card Settings" in "Speaker Properties" for Enhanced Bass and Trebble. Worked like a charm! Now I enjoy 100% A-Grade unaltered, Crystal pure sound.
PS.There is also settings available for 5.1 Surround Sound.
Hope this makes sense! Good Luck.