Mark Otto (http://www.markdotto.com/) is part of the expanding user interface team at Twitter, having recently worked at ZURB, an interaction design firm based in Campbell, by San Jose. Among the listed clients of ZURB where Mark contributed effort were TinyPic, the free image hosting service, and CC: Betty, the e-mail organizer.
Dan Webb (http://www.danwebb.net/) describes himself as a Web developer from London interested in DOM scripting and Ruby on Rails. He is also the creator of Twaud.io, a lightly used service aimed to bring audio to Twitter via MP3, through direct uploads or simple recording. (See also: @twaudio)
Twitter's Web and mobile interfaces have been relatively spartan to date, when contrasted with more media-rich clients, including Brizzly and Twazzup, which have moved to display video and images in line. I wouldn't make the leap that Webb's hire indicates Twitter would soon start supporting in-line play of audio, but his experience certainly can't hurt.