Sounds a bit daft to me - underlay on wooden floorboards then harboard nailed on top and then, presumably some other layer on top of the hardbaord.
When the underlay perishes - which it surely will - you will have to take up the top covering and then the nailed-down hardboard, which will rip.
Harboard levelling is a reasonable idea if your suspended wooden floor is uneven but all the floor layers I've used have fitted plywood for levelling.
Don't understand "soundproofing" - is there a rock-band in the cellar?
The whole scheme appears at first sight to be aimed at using up the left over underlay - the soundproofing and comfort reasons are your justification. I doubt you will achieve either.
Consider the top surface you wish to walk on and use the appropriate material between that and the floorboards - Underlay is designed to go under carpet.
Richard Buxton, September 2008