识 shi2 / zhi4 course audio correction
We are taught 识 very early in the course here, in the Greetings lesson. I did not notice this glaring error until I went back to review, because at the time of doing that lesson originally I didn't know any better. When 识 is taught here, it is only used with the word 认识 (我很高兴认识你）. So the audio should follow and be shi2. Shi2 is also the depicted pinyin in the lesson. However, the audio when you review the single character in the lesson is zhi4, an alternate pinyin for the same character, which has an alternate meaning, which would be like writing a footnote. There is not a relevant report option on these questions in the lesson so I figured I would bring it up here in the forums.
Native speaker. I think I can provide some insight on this topic and clear up some misconceptions people in the comments seem to be having.
识 is a character with multiple pronunciations, those aren't particularly rare in the chinese language.
[shi2] That's how it's pronounced in 认识 (to know sth., to meet sb.), or 识字 (literate, literacy, or lit.: to read characters). Note that 认识 is usually pronounced as [ren4 shi0], dropping the tone on the latter part of the compound entirely. This is the most common pronunciation of this character.
[zhi4] This pronunciation is most common in 标识 (to indicate, to symbolize). Though that word is most commonly written as 标志.
But I agree, Duo should get their pinyins right
A Chinese character (字) can be a word (词) or a part of a word. When it's a part of a word, if it's not the main part (typically the first character) its tone or even pronunciation could change. 识 as in the word 认识 has no tone. If you pronounce it with a tone, it would sound unnatural. If you pay attention, you will notice many such cases.
On the other hand, when a character is used as a word as 识 in 识字, it must have a tone. Similarly, when you read a character individually, it must have a tone.