The link doesn't work...
So, I don't know if the article you're sharing is saying that Tony Geiss actually performed the voices of those characters, or if they're using voice as an expression; to "give voice" to something is similar to saying to "give life" to something... for example, Jon Stone had such an impact on the show during the 70s and 80s with his directing, writing, producing, etc, that you could say he gave the show it's voice.
That said, we know Tony Geiss wrote a lot of absolutely wonderful songs for characters like Big Bird, Elmo, and Kermit, some of which really identify the characters and their personalities, that in that sense, he gave them their "voice" that way.
I'm sure that's probably not the case of the article of which you speak, but that is an example of how the term "voice" can sometimes be misinterpretated.
So, I don't know if the article you're sharing is saying that Tony Geiss actually performed the voices of those characters, or if they're using voice as an expression; to "give voice" to something is similar to saying to "give life" to something... for example, Jon Stone had such an impact on the show during the 70s and 80s with his directing, writing, producing, etc, that you could say he gave the show it's voice.
That said, we know Tony Geiss wrote a lot of absolutely wonderful songs for characters like Big Bird, Elmo, and Kermit, some of which really identify the characters and their personalities, that in that sense, he gave them their "voice" that way.
I'm sure that's probably not the case of the article of which you speak, but that is an example of how the term "voice" can sometimes be misinterpretated.