Just listening to the Cuckoo which also suffers a bit from the same - which is sad because the singing and playing is lovely.

Not being flippant but I'm presuming you're not standing in the soundbox when you are singing. Is the hiss in the background from the recording of the harp or a more general thing?

Can you not just mic a harp rather than have to put a mic INSIDE the soundbox? I would guess that the level of sound output by a harp must be at least that of my acoustic guitar - or is that not the case? The mics I have are pretty much the bottom end of the market (Altai UD-130: Altai are described as producing low end mics!) but they'll easily pick up the lightest strum of a guitar from quite a distance and don't have huge amounts of hiss or hum. Do you use some form of pre-amp to bring the level up to a suitable level for recording.

The other thing I did notice in a few tunes was that the peaks were clipped and a bit distorted here and there that suggests that the levels might be a bit high in some cases.

If you just record nothing with the mic do you still get the background hiss? If so might it be the lead, perhaps? A change of mic might make a radical difference.

Just my thruppence worth...