If the bitrate is 32kbps, it ought to sound EXACTLY LIKE AM HD. Same codec, same bitrate. At a lower bitrate, it ought to sound worse.
Actually there is a new codec technology that HD Radio is implementing so that 12KB/s HD signal when broadcasting voice can sound as good as 36KB/s. It was originally being used in cell phones to help improve voice communication trunk lines.
I have heard this. It does sound OK. I have received it in HD AM and it does sound good.
One thing, Master, and this is critical. Remember the large "B" as in KBPS means "Bytes". The small "b" as in kbps means bits. There are eight bits per byte, so 128KBPS is actually eight times more than 128kbps.