Bu2 ke4 qi4 is the correct pronunciation, in the meantime.
不客气！does not have audio pronunciation....
The audio file for 不客气 appears to be missing.
Is the sound for "You're welcome" not showing up for anyone else?
So bu ke qi as a whole is wrong but, bu keqi is correct
Bu and keqi are the two different words here. You see often in chinese that two characters together make one word.
should also accept 不用谢, right?
Yes, but that is colloquial. I've also heard 不谢
What does this literally translate to? I'm guessing some variation of "No problem"?
'No need for the civility, see you again' This is more or less the literal translation. The first part however can be seen as no problem or you're welcome, this is more of cultural reasons.
欢迎你 is you're welcome
Not in this context, 欢迎 is like a "warm reception"; "welcome aboard" The English here seems to be a response to someone's "thanks".
As far as I know, 欢迎 is used when you greet someone (welcome), and can't be used as a polite answer to 'thank you' (you're welcome). Correct me if I'm wrong :)
Usually "you're" is implied so it is "欢迎！" As a response to "Thank you", it is incorrect, as @Celticfiddleguy and @Lomochibi pointed out.
There is nothing in the hints to suggest this answer when you have the English and the characters to drag in - I am sure they are correct word by word but if the words don't translate to the phrase it makes it difficult to learn from.
不客气 : the audio doesnt work ! :S
The sound is missing for 不客气, This was posted a year ago and is STILL not fixed? Yet they have time to do an unneeded redesign of the app? Bah.. doulingo is losing points with me.
To be fair, in the context of your points being very valid, the new tips and notes section, available on mobile now, is an improvement.
I used the separate characters 不, 客 and 气 followed by the button containing 再见. This was apparently wrong...