The main difference is that the zenfone 9 is much louder and "cleaner" than the Rog 6. The zenfone sounds more crisp. It doesnt reach as low frequencies as the rog phone and the zenfones soundstage is also smaller.
Basicially the Zenfone 9 will satisfy most people quite easily while the Rog6 is a mess. It sounds wrong, muddy and while it has soundstage it really doesn't deliver in actual audio quality. I don't think anyone is actually impressed by the Rog6 speakers, while on the Rog5 on release you have seen tons of people comparing the phone to others and raving about the speakers, until ASUS decided to nerf the speakers because they would blow up the amps.