I agree with you @Gobee129
that it's kinda boring how his attacks take a long time to prepare. Just i think it's something that have to be like that.
Sound waves travel with great speed, if his attacks were instant he could be very dangerous. Even if he can't put haki on his waves, he still would be a nightmare for every other person different than logia user.
And lets really look at his Devil fruit. He is not fighting with music. I know that he use his body as instruments. He is fighting with sound waves.
And that waves can do many many things. They can cut things. They can confuse enemy's senses. I believe they can produce heat aswell.
I think sound waves with big frequencies can also create small quakes and tsunamis.
He can improve that in his DF, he can also improve the range of attack. I also think that he would be able to attack at all sides not only at one. That ofcourse would happen later in the story, because it really sounds overpowered.