I had a crackeling headphone socket on my H300 and it was really annoying.
I figured out that there was a something wrong on the inside.
I fixed it in about mins, Here is how I did it.
1) Take off the back of the H300.
2) Then when you get to the circut board you need to find the headphone socket. It is really easy To find.
3) Then Go to the place i have marked on the picture and with a little flathead screwdriver, you need to apply some preasure to try and bend the two little bits of metal down a little small bit. Not too much, just enouch to make the seal around the socket and the plug tight enough.
DO NOT PUSH THEM IN TOO MUCH!!!
4) Then plug the earphones back in and see if it is a nice fit.
5) But the H300 back together and then test it out.
This works in most cases and if the problem persists, try blowing in the headphone socket to remove and dirt or dust thet might be in there.
I, Sanitarium, Take no resbonsility to what happens to you DAP in the process in the process of trying to fix your DAP while following my instructions.