SOLUTION: muffled sound on Boot Camp on iMac built-in speakers.

If on iMac on Boot Camp you hear muffled sound from built-in speakers, devoid of crispy high frequencies, Steps that work 100%:

1) In Windows, reinstall boot camp drivers from USB stick that Mac OS Boot Camp Assistant creates. Reboot Windows.

2) Go to Settings - Sound - find Sound Control Panel at the right edge of the window (maximize windows if needed). There, choose speakers (Cirrus Logic in my case, green check-marked), press Configure (left bottom button), choose QUADRAPHONIC, next, check-mark both lines as full range, next, finish. Then, press settings button and see if the enhancement are NOT disabled! If disabled, uncheck, OK etc. You may once again reboot also.

Plus, one more lifehack: if you play your music back via iTunes, go to equalizer, enable it, move preamp slider to BOTTOM position (all the way down). In some seconds, sound will be much more quiet, if needed, adjust volume with system volume slider, iTunes main volume etc - built-in speakers will not distort powerful bass, if any, in your music. Even on higher decibels.