The whole game is just very quiet, the music is packing a significantly lesser punch than it could with higher volume. Can't be a hardware issue, other games sound loud enough with the same headphones. And don't give me that stuff about music volume increasing with rank, even if that's true, i personally didn't notice any difference in volume
I think 3 and 5 are tied. 3 has the better story and it has amazing weapons but some of those weapons aren't realized to their full potential due to odd choices like not letting Cerberus and Nevan have double jump, and only two melee weapons/guns and one style only at a time (wasn't their fault. PS2 hardware.ZBLm.