1. If the sound system hadn't failed, last night's show would have been better.

Question

a. this is a correct sentence using the 2nd conditional form
b. this is a sentence using the 3rd conditional form
Answer a.
3. if the club's manager agrees, they'll play a few more original songs next time
a. this is correct using the 1st conditional form
b. this is correct using the 3rd conditional form
Answer a
4. The duo could perform at weddings if Mike played the flute
a. this is correct using the 1st conditional form
b. this is correct using the 3rd conditional form
Answer b

Answer 1

1 is wrong. The other two are right.

http://www.englishclub.com/grammar/verbs-conditional_4.htm