I'm currently using a GoPro Hero camera for in-car video but the audio is simply horrible. The HI setting breaks up terribly and the LO setting can't even be heard without amplified speakers. I'm looking for a separate audio recording solution that I can use to capture decent audio in a digital format that I can then import into MovieMaker or the like and lay over top of the video. Twenty minutes of Googling hasn't turned up much so thought I'd ask here...

The one thing that did keep popping up was digital voice recorders but the descriptions make me think they're built to record meetings, dictation and the like and would be far too sensitive for motorsports use. Is this a correct assumption or are they more versatile than I think?

I'd look around (maybe ebay) for a DAT recorder and a stereo mic.
There are (were) several portable ones.
I recall seeing a site with all portable dat gear and mics on eyeglasses.
They used them to bootleg rock concerts with success.
I use an external stereo mic for my camcorder. The mount for it will wanna isolate from vibrations and harmonics.
I passed on this particular cam setup recently when my minidv died.
There are several sd card camcorders out there with megapixle receptors for right at $300.

jb