For Steam soundtracks that are sold like DLCs (or the free ones if they still require owning the game), you put "enclosure". For the price, I thought it had to be a sum of the base game and DLC but apparently
that's not the case so you put only the (initial) DLC price (it makes it easier, honestly) and in US dollars preferably (or another currency if it happens the thing isn't sold on the US Steam... but anyway, yes, put USD if you can because there isn't a multi-price option).
I did come to a realization that a new 'DLC' category would fit better instead of 'enclosure' but who knows if that will be implemented or not so you can create Steam DLC OST entries using the current way.
As for your point no. 2 (HMH2), yes, it's acceptable (still as an enclosure).