Here are a few links that give you a taste of what HD voice actually sounds like. It is worth noting that not all 'HD' Voice calls sounds the same. While all cases include some kind of wideband VoIP codec, some implementations include extra enhancements like specially designed hardware or improved acoustic software technology.
AudioCodes is a VoIP networks and technologies company. Their implementation of HD voice is called VoIPerfectHD. According to their website, this implementation "relies primarily on AudioCodes' leadership in DSP (digital signal processing), voice coding and voice processing technologies"
Polycom is a telepresence and voice communications solutions manufacturer and seller. Polycom was actually the company that trademarked the term 'HD Voice'. Their implementation of improved call quality includes wideband technology, their patented Polycom Acoustic Clarity Technology 2, as well as the "best software components and hardware design for maximum sound quality".
The following link features a clip from the Polycom 650, one of their executive class IP phones.