Boom box.

A water feature does a lot to cover neighbor noises we have a fountain in front and used to have one in the back until we installed a pool with water jets. The jets were expensive, but love that the neighbors can not hear us talking and I can not here them. The water feature does not need to be large, but the water needs to fall at least a few inches we used a piece of copper tubing to extended the bubble height when we had the smaller backyard fountain.

Invite your neighbors over for a glass of ice tea and cupcakes outside right by the fence where you usually like to talk. They will be shocked at your hospitality and probably wont want to eaves drop again because they feel bad.

Just make sure that you play something obscure like yodeling or Gregorian Chants

Then you find out it's their favorite music.

How about a wind chime or two?

Or, for the next week, go out and sit but don't talk at all. They'll probably get bored:)