Audio is an integral part of web conferencing. There are two ways of providing audio service: either a vendor offers an integrated VoIP solution or they offer POTS (plain old telephone service) aka. landlines.
The quality of VoIP is generally very good and exceeds that of a standard phone conference by far. And integration of VoIP should actually be no biggie for the vendors since they simply need to add the audio to the web conferencing functionality already in place. Yet, mostly phone conferences are used for web conferencing. How is that?
Well, even though with broadband easily accessible to deliver the VoIP, many users of online collaboration tools are missing one integral part of the solution: A decent USB headset or computer mike. And many users also feel it is easier to pick up the phone and dial into a phone conference in parallel to the online meeting. Big vendors clearly have an advantage here since they have the resources to provide an infrastructure of geographically dispersed audio bridges to even enable this functionality.
There is a trend in web conferencing, however, that shows an increase of VoIP usage. The increase is slow, but it is noticeable. Mobile devices play a role here because e.g. on an iPad you need VoIP since users will hardly be sitting next to their office phone when participating with an iPad or smart phone.
What will happen in the near future is the following: while VoIP clearly is the future, vendors will provide both POTS audio service and VoIP. And they will have to, because offering only one will equal a huge loss in revenue simply because too many users will be shut out. To get users to adapt to VoIP quicker, vendors should maybe consider bundled offers of web conferencing services plus the necessary peripherals such as USB headset and webcam.
And yes, if you have a fairly new computer it is likely to have an included mike that delivers a decent sound quality as well as a webcam. But this is not the case for a vast majority of office computers. And until that changes, adaptation to VoIP will remain a slow process.