Are you wanting the sound to match up with each letter as it displays on screen? Is your problem creating the sound? Is your problem matching the text to the sound?
A little more detail would help.
Not near a computer, so I can't remember if Vegas has a "bulge" filter. If so just do 5 frame keys: Bulge up; bulge down; bulge up; bulge down.
If not, a simple scale up; scale down will accomplish the same end result (more or less) -
some audio assist will make the effect a million times better - anything from actual breathing, to a musical hit, to a slide whistle - whatever; as long as it's in sync with the video, and repeats.
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You didn't say much about anything. I wasn't trying to insult you, but I did not understand what you were asking for so I asked for more information and attempted humor at the same time by playing off your question. Apparently, that did not go over well with you.
When you said throb- I thought of a stethoscope and the sound a heart makes or when you see a cartoon charator hit their thumb and the noise it makes. That is what I was thinking but was unsure as to what exactly you wanted to do. Good thing or else I would have wasted my time.
You can check out swishmax for 30 days for free. There is a tutorial on a typewriter effect on VASST.com that can give you the basics on how to use the text features of swishmax, even though it is geared for the typewriter effect. I would give it a try as it will do what you want without worrying about keyframes etc, that is unless you want to mess with the keyframes [EDIT:(and you can do it within a fraction of the time that you can in Vegas IMHO)]. You can import the saved flash file right into Vegas and plop it on the timeline.
you also might try using a height map with moving water beneath it, and time the water movement, or for that matter, create a generated media file to create the mapped movement. That'll work great.
See how a nice, simple thanks brings out the best in people?
You might try applying the spherize Video FX to your text, and then keyframe it accordingly. Work with the leading, kearning, etc. of text to get the look you desire. You might try adding some glow effects as well. I've used this method before and it works very well, plus you don't have to pay for another titler.
Oh... you didn't ask me about glow ;-)
Good luck with your project.
Easy gents. A true sign of maturity is gratitude.. This forum is the best I have seen and a toast to gents in it. For the information, the advise and the humor none other has come close. Thanks to all of you.