PUZZLE 108 Now i Know my ABEFs

There are five metal plates pinned up on the wall.
What do you think should appear in the place of the ? in the middle?
Look at the six shapes below, and put two of them together to make the shape that takes the place of the ?.

Hint 1

Trying to make letters isn't going to go well.
You should play around with the shapes and see which combinations you come up with.

Hint 2

The '?' is replaced with two letters.
Look at the letters around it to figure out which ones.
However, you cannot make letters out of the shapes available.

Hint 3

The '?' is replaced with something relating to sound.

Super Hint

Try making the shape of a CD (Compact Disc).

Answer for 'Now i Know my ABEFs'

The answer is '1 and 3'.