It's the drivers, there is no fix, and it's been years without a driver update. Asus has no intention of even acknowledging the existence of their own sound cards, let alone fixing their broken drivers.
You can go to the Headphone Settings and set the Gain to medium (seems closest to speakers) or high and it will be loud enough, but if I understand correctly (someone correct me if I'm wrong) using the Headphone output bypasses the Realtek ALC1220X codec/dac and you lose the advertised low noise:
"Realtek ALC1220X codec delivers a crystal clear 116dB signal-to-noise ratio (SNR)"