vb wavelib
I've used WaveLib to capture audio from the line input. You can search for it and find many examples. The advantage is there is nothing to install - it gets compiled into your code.
If you decide to use the raw register interface to your sound card, your application will not be portable. That means it will only work with one sound card.
If you elect to use an sdk like DirectSound or WaveLib, it will be portable and work with any sound card since each sound card will have it's own driver installed and provide a generic interface.
When testing, make sure you setup the Audio Mixer correctly (most default settings will turn off line input in favor of mic input) or you won't get any audio even if your code is working.