Well it isn't actually a confirmed case. Ruiu, the original person reporting the issue wasn't sure exactly what the surface area of attack was at the start. Ruiu Dragos, who is a security researcher believed it infected via speakers.

Eventually Errata CEO, Robert Graham, said that if he spent a year, he could build malware that did the same and that it was 'really, really easy'

Eventually, Ruiu noticed that the initial stage of infection was from one of his USB sticks.

The speakers part comes in that he found that the packets transmitted between badBIOS infected machines stopped if he disconnected the internal speaker and microphone.

Meaning, that sure, badBIOS may communicate data with each other via speakers but that it has never been proven that it could actually infect another machine via speakers. However, that hasn't stopped articles from conflating things.