1a. That house still hasn’t sold.
1b. That house still hasn’t been sold.
2a. Their first album sold more than a million copies.
2b. Their first album was sold more than a million copies.
Please correct the above.

2b is incorrect, but you could use ‘has’ instead of ‘was’. The others are all possible within the correct context.

I think ‘More than a million copies of their first album were sold (out)’ can be acceptable.

Not quite , ‘sold out’ is unacceptable there, but anyway the original didn’t need correction beyond the single word indicated.

But if someone wants to put it in the passive, won’t it be all right without ‘out’? You see, the intransitive use of ‘sold’ often creates a sort of confusion in foreign learners.

The fact that it is correct without ‘out’ does not mean that the correction was needed. Especially as the changed word order changes the emphasis of the sentence.

It’s not a correction; just a recast for those who would need to know! LUSH.