Can a computer transmit packets via high-frequency sounds broadcast over speakers ?
In theory yes, it's the same as transmitting over analogue phone lines. Packets of data could be encoded as pulses and tones to be broadcast over speakers. As most speakers can generate high frequency noises which are beyond the range humans can hear, I see no reason why this couldn't happen.
and wondered if this attack vector was possible ?
The concept of receiving the packets is slightly more complex in that it couldn't be the initial vector. As indicated in the article, the initial infection was likely due to an infected firmware in a USB storage device.
I would theorize that the virus isn't transmitted via sounds initially, but is using audio as a method of command & control in environments where networking is either not available or severely restricted due to security measures.