This is tricky but here are the possible scenarios:
1.) Wheeling alone on rougher than expected trails: I listen to the sweet sweet music of my knuckles cracking as they turn white on the wheel and I am praying to a god I don't believe in to not let any part of my truck break and strand me.
2.) Wheeling alone on less rough roads: sometimes Dad Rock (read: Classic Rock that reminds me of my Dad who used to take me off-roading in his '94 4Runner back when it was new), sometimes alt-country stuff (Moondoggies, Maldives, etc.), sometimes black metal, sometimes thrash metal, sometimes doom gaze.
3.) Wheeling with anyone else: whatever the **** I want to listen to because they can get out and walk up the damn mountain if they don't like it. Just kidding, luckily most of my friends and my significant other have similar tastes in music.
As for windows down, so much dust! I tend to sunroof it (unless brushing under tree branches because I don't want bees/arachnids to fall on to my head...ask me how I know) and leave the rest of the windows cracked or up.