Here's where I make a number of vendors angry.
I am seeing a growing trend in the exclusive use of computer-based audio for webinars. I'm not a fan. Let me be perfectly clear what I'm opposed to, however... I have nothing against offering streamed audio (aka: computer audio or VoIP). I think it's a fantastic option for your audience and I always prefer giving my listeners a choice of hearing their presenters via telephone or computer. In recent webinars where the audience had such options, I have seen anywhere up to 75-85% choosing to listen over their computers.
But if I am forced to make an either/or choice of one audio delivery method, the phone wins every time. Computer-delivered audio can work very well. The majority of your audience may have a perfectly acceptable experience. But over the course of many, many webinars using a variety of web conferencing technologies, I have never hosted or delivered a webinar with streamed audio where at least a few audience members didn't complain about the sound quality. It's inevitable, given enough people listening.
Some audience members have slower computers, or they are running applications that conflict with the audio reception and playback. Internet connections get busy and slow down. Data packets get lost and have to be retransmitted and reassembled. Some people have terrible audio cards and speakers (many rely on the cheap internal speakers built into their laptops). There are too many potential problem areas to ensure that your entire audience will receive a quality, uninterrupted broadcast.
As a technical moderator, there isn't much you can do when someone writes in that they can't hear the audio cleanly or at all. Diagnosing and troubleshooting an audio reception problem is impossible when you are working a full webinar audience. That's why I love having the fallback of "Why don't you just dial in to the telephone conference line." It's quick and easy.
Some vendors force the presenters to deliver their audio via a computer-connected microphone. This always makes me shiver when I see the requirement. I know what's coming... cheap, low quality computer headsets being pressed into action (or worse: stand-up desk microphones). Attempts to configure audio properties by people who aren't used to dealing with such things. Technical troubleshooting sessions examining issues of compatibility with operating systems and input devices. Ugh. When I am hosting a panel of guest speakers, letting them use their comfortable old telephones gives me one less item to deal with.
There are several benefits to using streaming audio, and I appreciate them. The cost can be much lower than providing a telephone conference line, especially if you have an international audience. Participants away from a convenient phone line can still listen in. It does away with an additional setup item for you (scheduling the phone conference line) and eliminates an additional set of instructions for your audience on how to connect with the phone conference. Those are all good things.
Streaming audio is also the best way to work with an embedded video clip as part of your presentation. With a phone conference, you can never be sure when your entire audience has finished seeing the video playback and you can start talking again. Some will be done early and listening to silence, while others are angry that you have begun talking over the end of their video. When you deliver your audio over the same integrated data stream as your video, it all gets buffered together to present a smooth flow to your audience. Sure, they might be anywhere from half a second to ten seconds behind you, but at least they get all the content in the right order in a steady stream delivered at the rate their machine can deal with it.
Unfortunately that same buffering delay factor can cause problems in interactivity with your audience. If you issue a poll or invite audience responses via feedback panels or typed chat, you want to react in real time to their inputs. If you have to wait for ten seconds after you have asked them to participate in order for them to hear your request and take action, it creates an uncomfortable dead spot. You can talk through the waiting period, but it tends to slow things down and drags out the very parts of the webinar that should be the most stimulating for your audience.
As with everything else in the webinar world, there are tradeoffs in the way each vendor implements their technology and your priorities will drive your selection. Hopefully this note has given you some pointers that will help you make an informed decision that is right for you and your audiences.